They are one source of drama due to people's opinions and feelings. Forums is a place to express people's opinions and sometimes people take it to personal. So then that usually turns out bad for both sides of the arguing parties. It doesn't have to be, though. That's just the way it is.
I think playing the game is the number one source of drama. If someone doesn't like how you play it, they will fight you for it. I seen it happen many times.
I agree with Marty. It's not just the forum that brings the drama out, but it is the whole game that brings it out. Like what was mentioned, it just tends to be brought to the forum to get attention so it is seen a lot.