I think it really comes from the ease of the exit and the disconnect from anything human. Even when you come down to it on a snail-mail level there is still a human element to that communication. Whether it be the signature at the end, or hosting a physical object in your hand, I think something makes it more real.

When you are online everything fits into the same type of frame: Words in a font on a screen. It looks just the same just about everywhere. I think it may be that these people believe they are yelling more at a screen and some text than a real human being on the other side.