As someone who's been involved with fandoms for twenty years now, since the pre-LJ era, one of the most baffling and actively enraging developments in recent times has been the sharp rise of content-policing from within fandom itself, rather than something caused by the intervention of an outside entity (as was the case with the fundamentalist Christian group whose caterwauling kicked off Strikethrough). I've read and written reams of social analysis when it comes to the whole 'anti' phenomenon, caused by their actions that very much took the shape of actively coordinated campaigns against some of the content that I enjoy in fandom (chiefly non-con fic and incest ships). 

Kickstarted by my bitter disappointment in running into an old friend on here and seeing them turned into another 'how dare you engage in rape/incest/abuse apologism!' zealot, here are a series of comments I left on Reddit, a while ago, explaining the social roots behind the whole anti phenomenon when someone brought up the thing in r/HobbyDrama, explaining exactly how, in ten short years, fandom went from YKINMK to.... this complete shitshow we've got on our hands now. 

What even led to the appearance of the whole 'anti' thing? Several factors: 

• social justice -- the most bastardized form of it, present on Tumblr. What happens when 
  • you're got a mass of young people who have no clue where to direct the anger they feel at a world perceived as wholly unfair and 
  • a system which teaches them that the most oppressed person in a given situation is the one who should be listened to and always have the final word? 

The answer is weaponized victimhood or malicious self-victimization, whichever of the terms you like best. This is why a metric fuckton of SJ-based arguments on Tumblr just devolve into a complete morass of 'you should listen to me, I'm X and have been through Y', 'no, you should listen to ME, I'm X and Z and have been through Y!' The recent apotheosis has been that, when two people who have gone through the same thing (say, two survivors of rape) meet and disagree over the merit of nonconsent kinks and catering to them through the safety of fictional content, the person against content-elimination will invariably get told that they're not a 'real survivor' or that they 'secretly enjoyed it.' How can someone be so utterly heinous as to say something like that? Through the wonders of malicious self-victimization -- the anti knows that they have to win the argument, at all costs and they do this by positing themselves as the most unequivocally 'harmed' party in the conversation and the only 'real and proper' survivor. Because that, in their own filter-bubble, is what gives them the most social capital and the most support in the argument. It doesn't matter who and what has to be trampled underfoot -- all that matters is that the anti 'wins' the argument.

• bastardized social justice bleeding into fandoms -- take all of the above wankery and apply it to transformative fandoms. 

When I was growing up, the appeal of transformative fandom was specifically the 'anything goes' aspect. You could write whatever fucked-up nonsense your hindbrain chose to come up with and aside for a few people wrinkling their noses at you, you'd largely be fine, because it was a widely understood fact that writing about something had zero relation on whether or not you'd go and do it IRL. Hardly anyone thought a darkfic writer to be an actual rapist or a serial-killer or, as is the most prevalent accusation right now, a pedophile. I remember winning a darkfic-writing contest as a young teen by writing about someone murdering Admiral Motti of Star Wars fame mid-intercourse. With a spoon (it was much gorier than it initially sounds). Back then (over 15 years now) I wasn't accused of 'fetishizing abuse', I wasn't interrogated on my fictional tastes and how they related to my personal morality. All of that came gradually, in the last ten years -- it started in fannish spaces on LiveJournal and gained speed on Tumblr, to the point where, as early as 2015, I was getting screeched-at if I defended darkfics ('she just wants people to let her write her gross rape-fic in peace, how dare she?!') What happened was that fictional harm (and the personal discomfort it could engender in someone consuming the content) was 100% equated to real harm. Thus, me writing a fic with sexual violence in it was, for antis, the moral equivalent of being exactly like whatever individual abused them.

• (induced) hyper-fragility -- the logical end of what I was talking about above. 

Current fandom consensus is that you're supposed to tag and warn the shit out of your content, if it contains anything that could be objectionable, so people who don't want to deal with that can just avoid it or vamoose it entirely via blacklisting. How does that square with antis howling even louder than before against 'bad content?' Simple. They don't want tools to curate their own fandom experience -- you could spoon-feed them everything and they'd still scream. Their goal is the total elimination of whatever content they've fixated on (age-gap ships, incest ships, content with sexual violence, BDSM content, etc) because even the mere knowledge that this content exists (and is being actively and freely enjoyed by 'gross people') is enough to put them into a bona-fide emotional meltdown. We've got what looks like an entire generation that effectively brainwashed itself into thinking that it is so hurt and so fragile that it needs everyone else to mindlessly do whatever is demanded of them, for the protection of this particular group. For example, I have a number of ships and tropes that make me go into a mental-health tailspin. And yet, even as a teenager, I always acknowledged that my personal discomfort and personal hangups gave me zero right to demand that others stop producing said content. That was just the fannish social environment I was raised in, which again makes me think that this is largely a generational thing -- I've yet to see anyone 30 or older holding the views of 'antis'. 

Nota Bene: Not everyone is actually sincere about something sending them into a meltdown. Among antis, there's a hard core of ruthless social manipulators, usually older than the predominantly teenage antis, who are cynically using the 'movement' for their own gratification and power-plays (they're basically Tumblr fandom's toxic, cult-gathering BNFs).

• filter-bubbles -- I've mentioned them before, but they're responsible for much in the way of people's ideas and 'discourse' on Tumblr becoming more and more extreme and detached from reality. 

In the case of antis, they specifically congregate in their own tags and have formed their own social community, where it's a shunning offense to express even a modicum of agreement with any ideas that run counter to anti thought. Hell, I've seen people being made-an-example-of purely because they remained friends with individuals who did not subscribe to anti views. The goal is to isolate the (usually young and inexperienced) individuals and make them socially and  emotionally dependent on the anti community, to the point where they don't dare question anything, for fear of ostracism or outright targeted harassment. This combines nastily with the wider Tumblr social environment's habit of locking people in a death-spiral of more and more extreme versions of initially sensible ideas. 

A concrete example -- sensible, rational posts such as those warning teenagers not to be too forthcoming on the Internet with personal information, on account of potential predators had their essence passed down into other posts... that over the years got more and more warped, to the point where we now have people shrieking that any adult having any interaction with a teenager on the Internet is a predator and an active danger to kids. Also, that adults who provide science-based, fact-based sex-ed to teens from ass-backwards states or societies are now doing it to 'groom' said teens. Progressives talking identically to US American religious fundies? It's more common than you'd think! Another example: posts warning teenage girls of potentially predatory older men over the years got warped into this fucking gem: '[college] freshmen, don't date seniors! You're still a baby-adult and they're targeting you because they're abusive and want someone to easily manipulate!' Yes, I had to read the words 'baby-adult' with my own two eyes and now you have to as well.

• goddamn ship-wars -- what I said above, about people cynically using the whole 'anti' thing to their advantage? This usually manifests in fandoms as people using SJ lingo in order to win ship-wars. 

In Ye Olde Fandom Days, ship-wars usually involved people calling each other 'idiots', 'cretins with no taste', etc, for not shipping the insulter's Precious Preferred Ship and supporting the rival one. On Tumblr, however, a lot of individuals wizened up to the fact that such insults were utterly passe and just bounced off their target. If they wanted to actually hurt someone (and do it in a Socially Acceptable Way), the best way to do it was to accuse the other person of supporting a variety of '-isms' / of engaging in oppressive behavior by liking the rival ship. The funniest thing? In Tumblr fannish spaces, largely dominated by women and other marginalized groups, such as LGBTQ+ people? We bought into their shit for a while, because we were earnestly trying not to do harm to others and were taught obsessively that if someone said we were harming them, we needed to listen and take their criticism on-board. That's how a ton of anti nonsense is, nowadays, being used as a front for ship-wars -- 'my ship is progressive and not-straight and interracial and healthy and supportive, your ship is [insert the opposite here... even when it doesn't correspond as a description for the ship!]' This is also why there's a veritable rat-race going on to 'problematize' even the most anodyne ship and fictional content, because that's the most surefire way to use it as a bludgeon against someone else.

The most chilling aspect of this whole thing is that this fuckery has bled out of fandom and affected people's real lives. And the reason for this is precisely the unholy mix of Extreme Tumblr SJ and fandom, one that puts people in a filter-bubble which teaches them that they are RIGHTEOUS in their goals and their methods and they mustn't question themselves or the legitimacy of their targets. The Righteous never question, the Righteous are always correct and if they're told someone is a Suppressive Person* Bad Person, then they must cut all ties with them, publicly denounce them and, if they continue in their Badness, punish them appropriately.

^{*Did I just compare Tumblr SJ to Scientology? Yeah, I did, sue me.}