Those fandoms -AT, Undertale, Homestuck, SU, et al- fall under the umbrella of what's called the "
Migratory SJ Fandom". It's basically A Thing that has elements that appeal to the SJ crowd and they flock to it like locusts, and in the process, make everything absolutely insufferable for the rest of us who, even though there's acknowledgement of any problematic aspects of it, just want to enjoy the thing. When the source material has done something problematic in their eyes, they disavow it and move onto the next thing.
From my understanding
(and without powerleveling too much, I've been active in media fandom for years and have only recently begun a slow extraction to get out from it), the vectors for these in the modern era are considered to be either Homestuck or Dragon Age 2. Both of which began to take flight around a fight that's called '
RaceFail'. (I've also heard Hetalia was one, but that's debatable) As Homestuck began to wane in popularity (it's nowhere near the juggernaut it used to be, that's for sure), its fandom then began to spread to anything that looked remotely "progressive" and bring their extremely warped views of social justice with them. Dragon Age's fandom largely stayed in BioWare circles initially, but began to fan out to various AAA titles on either PlayStation, PC, or Xbox. There might be an overlap with Undertale, but it's hard to say at this point, 'cause UT's fandom is mostly made up of ex-Homestucks.
The whole 'Superwholock' phenom is sort of its own thing (not counting series like Teen Wolf -which is deader than dead now, but was a SJ-heavy fandom for a while there- or the Marvel Cinematic Universe), but it's really died down in the last year or so, as a lot of its parts have fallen out of favor with the fanbase in recent years, but back in its heyday it used to be a SJ wank powerhouse. Last I checked,
that SJ-y segment of media fandom has glommed onto Star Wars in the wake of 'The Force Awakens' and I'm pretty certain they'll move onto either Suicide Squad or Batman vs. Superman next. Yeah.