Say, for example, you post a picture of a cute kitten on alt.cute-kittens. Seems pretty innocuous, right? How could this possibly be the source of internet controversy? Well, like clockwork, someone will claim you are not treating the cat properly, or argue that inbreeding purebred cats through kitten mills is harmful for society, and then some PETA jerk will chime in with "animals should not be kept as pets" and so forth. And then a troll will post something to try to get you to react. Two people commenting will start a flame-war with each other, each claiming to be more expert on the topic of cute kittens or whatever.
Youtube has similar problems. If you want to preserve your sanity, never read the comments on YouTube. You will walk away profoundly depressed for a week. How can people become so angry over cute cat videos? Yet it is possible. Cute cats are better in Mother Russia! Gosh, I wonder who posted that comment?