Aptly enough, there's a recent Adam Kostko blog post about that dynamic:
"I’d contend that this problem is a direct result of the immediate experience of the internet. We all tend to experience the online space as an intimate and personal one. Everything there is somehow “for us” — and if it’s unwelcome, it’s an intrusion on us...
Meanwhile, even as we are experiencing everything as taking place within our most intimate personal space, the personhood of the other is stripped down to bare text. What is important here is not simply that the face-to-face encounter that would constrain aggressive behavior is absent, but also that this text is an invasion of our personal space. If it’s too long, it’s an imposition. If it isn’t responding to our messages in a way we feel is appropriate, it’s aggressive or offensive.
When people have this gut reaction, it’s not necessarily because they’re bad people or impatient or overly touchy — it’s the inalienable structure of the internet experience as such."
http://itself.wordpress.com/2012/06/06/a-phenomenology-of-online-assholes-with-practical-tips/