There has been very little good empirical research on this topic, despite it being an issue now for nearly 2 decades I'd say (starting back with the Usenet newsgroups).

The short answer is simple -- there are a great variety of personalities in the world and before the Internet/online became commonplace, we were exposed to only a small minority of such personalities in our lives. We tend to choose to not be around people we don't like/don't get along with, and that usually works in almost every face-to-face situation (except family, which is the usual exception for almost everything).

Online, however, we have less control over how we get exposed to people we don't like, and people who have personality or control issues take advantage of this dynamic. The Internet gives everyone more power (to get their voices heard, for instance), but at the same time hasn't always kept up with giving others the tools to ignore those just looking for attention or control.

Since there will always be people seeking attention or control, the Internet has simply made it easier for these people to be rewarded and reinforced for their behaviors.

Hope this helps.

John