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.