Lack of empathy (Hard to feel empathy on the Internet; you can't SEE the effects of your words on what are essentially just more words on your computer screen).
Lack of accountability (People have said things to me on the Internet which would have earned them a punch to the head if they had said it to my face
and I suspect they knew it).
Lack of identity. Sociopaths can go where others fear to tread. Or something like that. In other words if you let everybody in on a forum, you're going to get everybody, including sociopaths and troublemakers. Ban, them, they'll come back under another alias.
Lack of impulse control. (Think twice before typing once was something someone told me once upon a time).
There are other reasons of course, but basically people are ornery and if you put a bunch together they'll behave badly. At least this is what history teaches us.