a number of subreddits like r/ConsumeProduct were taken over by the hate mongers from the_donald. You would have post brigaded to the front page that had absolutely nothing to do with consumerism or product promotion
Social media has a trolling/brigading/hive mind problem in general across all platforms and I would add solving this problem to Paul Graham’s list of “frighteningly ambitious” ideas.
I'm fairly certain it started off that way. Unfortunately it doesn't actually take much coordination to brigade and take over a subreddit.