I often wonder about that. I think in some cases it is the true personality coming through, but at other times it's just that people get caught up in the fray because there are no direct/immediate consequences. Not so much that they are frightened hypocrites, but just that it is easy to write things on an anonymous keyboard. There's no risk of someone knocking their teeth out.
That said, I know someone who wound up getting harassed (stuff was sent to their employer trying to get them fired, threatening phone calls at home, etc) because of something written on a forum...that seemed innocuous at the time. But the person on the other end was a crazy. You never know what you're dealing with.