Chan-type sites very, very strongly encourage right-wing social behaviors by nature; it's a constant struggle for social dominance on a post-by-post basis. Survival of the wittiest. All interactions are throwaways, and there is no reputation to maintain. So of course as soon as anyone gets frustrated, the filter goes off and the prejudices come out. Ad hominem after slur after flamebait. It's a feedback loop from there. Heavy-handed moderation can make it slightly less bad, but only by so much.

There is a meme something along the lines of "given enough time, any community will turn right-wing without heavy left-wing moderation".

I'm inclined to agree; it's not about the website format so much as how the admins/mods handle the community. See Twitter vs the various twitter clones, or reddit vs communities.win - the websites are designed almost the same, but the moderation policies are totally different, so the communities end up totally different.