I think it revolves around attitude. Some people hit forums to get a wide scope of views or opinions. Some use forums as a podium, here's what I think listen to me. Others are on forums with a more general if I don't agree with this post I'll say so, if I do agree I'll say so too. Others like to add their thoughts and just mix it up with folks. And then there are those that simply lurk.
So why so many arguments on forums? "Cause no one knows who you are and they can't see you and so you can say whatever you want, which normally face to face you never would. Anonymity brings out the lurking you on the internet. It's actually very interesting stuff.