Gabriel's Greater Internet Fuckwad theory explains it rather well.
The internet makes it very very easy to get your subject out. You can very easily put out a hate-filled message on a whim - it only takes a few minutes to get an account at an average forum, for instance.
Out in the "real world," if you want to put your ideas out in public, it's much harder. There's just so much to add to your inhibitions.
But on the Internet, you can do anything you want to without coming into direct contact with anyone. That makes it easier to be a dick, I think.