I've noticed this problem with a lot of forums that have a large member base. I think some people feel less inhibited in the forums where there is a large member base and significant amount of posters because there is a degree of anonymity and the feeling that they can say what they want without repercussion in the "real world".
Then, there are those people who seem to live off conflict. And, what better way to be involved with conflict than in a public forum where there are hundreds, if not thousands of varying opinions.
There are also people who seem to not have a firm grasp on language and/or an inability to correctly put into words their opinion/s.
And, of course, there are people who are just mean. I was a member of a forum years ago for stay at home moms...Man, the rudeness on that forum! I quit going. It turned into nothing but battles between stay at home moms and working moms, moms criticizing other moms over their methods of child raising, women bad mouthing other women because of the clothes they wore ("they shouldn't dress like that, they're moms!"). It was terrible.
After years of spending little bits of time on various forums, there are only 2 I still visit. KH, of course, and a forum a good friend of mine runs and I'm an admin at. At both forums, the drama isn't there and there's a true sense of community and "family". Of course there are disagreements but the majority of the time we all try to post our opinions in a way that won't offend another member because of the feeling of "family" that has developed.
One last thing I've noticed is it seems the more specialized a forum, the less conflict. The forums that are based on a single common interest, such as KH, seem to be more friendly than general interest forums or forums with many various topics. They seem to lack the bond that common interest forums have.
Anyway, that's my take on it!