I do prefer the 4chan theory, since it satisfies Occam's Razor. That said, the frequency of new videos from multiple channels dedicated to nothing but these videos, as well as the monetisation, makes it unlikely. Also considering the number of subscribers the subreddit has I'm sure that if there were any new 4chan threads suggesting people make those videos it would've been spotted.

It's honestly the sheer number of videos that is the shocking part of the mystery. It's not just a few dozen videos from different animators, it's hundreds of videos from dozens of channels pumping this stuff out. Unfortunately I find it unlikely that we'll ever find out the cause.