I've seen this in the European-focused subreddits lately. Places like /r/europe are usually pretty "normal" discussion boards considering that their demographics are English-speaking, internet-using young people of those regions. But periodically the comments (and comment scores) end up very uncharacteristic in a thread or sub-thread, especially if it has to do with immigration, Islam, or nationalism. Not just in the sense that some people have conservative or nativist views on those subjects, but that suddenly the discussion seems to be totally dominated by extreme versions of those views, anything vaguely liberal said about the subject is downvoted to infinity, etc. And the reason in that case if you dig around usually turns out to be that someone in a far-right/xenophobic subreddit linked there and caused an influx of people with those views.