Why it happens doesn't really make much sense to me either

A lot of the time, its started by someone with some racist, sexist, or otherwise socially unacceptable view points. They want to share and discuss their view points, but everyone they hang out with thinks those view points are unarguably wrong and only idiots would possess them. So they convince the people around them that they should make fun of those people. What's the easiest way for non-comedians to make fun of people? Mimicry/mockery.

Now original dude with hateful views has his friends parroting those same views, but its all good, because they're doing it for the luls, you see. In a bubble, like a group of irl friends, it doesn't really go anywhere from there. On the internet, where nothing can be contained in a bubble, people who share original dude's hateful views sees a group of people sounding off all the hateful things they believe in, and see a safe group of people to hang out with and join in. The people doing it as a joke just see someone who's joining in on the joke, because no one would believe this awful shit, lololol, and they all become friends. This happens ad infinitum. For the luls.