It happens if a niche reddit has a popular post and makes it near the front page. Or, someone makes a post that gets the attention of one of the brigading subs(even if mods remove post as soon as they see it, it might be too late). Usually the sub will have an influx of low quality posts/shitposts for a while after that.

Also, not subreddit-subreddit, but multiplex times, posts of mine got reported to admins for hate speech because I said "retarder" in a mechanical focused subreddit, and I suppose they don't care to take the time to realize it's a perfectly fine piece of technology and not an ableist slur before sending out threats of banning you forever from the site.