Part of that is not that people behave worse but that communication in writing, without facial expression and voice tone and the like, is prone to sounding a lot harsher than it likely would face-to-face. Especially if someone is prone to hyperbole, sarcasm, irony and so on, the lack of voice tone and facial expression tends to have a huge negative impact on "tone".
That's also true ... I'm a practitioner of sarcasm myself and have to be very careful not to use (overuse) it on the Internet. But if you work hard enough, you can convey tone through your writing.
I think the two other culprits are the de-personification that can so easily occur in our interactions and the tendency to interpret statements with the worst possible meanings. This problem never seems to happen when the person is someone we know in "real life".
Enough with the thread hijacking :) ... and if this gets much deeper I'll have to consult with my psychologist wife as I'm sure there are proper terms for most of these behaviors. (And yes ... I am her live-in case study)
