It is not really that weird, when you think about it. Sadly, I think it is that gang mentality where if one person gets away with a rude comment, the next five people all think it is ok for them to jump in. Also, if people are being bullied or feel lonely in real life, they might want to take their aggression out on the forum whipping boy/girl. There are a lot of people out there who think it is cool and brave to kick people when they are down, and they is no better way to make your community feel tight-knit than to start excluding people.
Anyway, those are just some of the reasons it might happen.