It could be the fact that annonymity and not having to look people in the face allows posters to insult other users without any sort of guilt. If you can't see the person's face, you don't know how hurt they are by your comments, and you can easily ignore their ripostes. It's having your cake and eating it.
Also, with conversations being in print, and with time delays, people often take offence where it's not intended, and fill in the conversational blanks in a manner that suits their mood.
People bring a lot of their personal baggage onto message boards too. So if you're having a shitty day you can snap at someone online and feel good about yourself, rather than shouting at a friend or spouse and having to apologise. This also comes out when people go on huge tangental rants in their posts because they have no outlet for their ire in the real world. No one will cut them off in the middle of a paragraph, and someone, somewhere might listen. They've wanted to say this thing for ages, but there hasn't been the slightest opportunity in real life, so they shoehorn it into a thread via the most tenuous links. One might even identify another poster as "symptomatic" of a social attitude that you despise, and choose to attack them online to get it of your chest, rather than actually dealing with the problem, or the people you know who are guilty of it.
P.S. - I am usually this long-winded in the real world.
