My thinking is along the lines of UserXYZ posts something humorous and gets 150 points. This causes others who feel they have a funny bone as well to pile in and attempt similarly witty remarks (which are initially harmless or contribute positively); but then you have an avalanche of users who become aspiring comedians on the message boards they frequent. I sometimes feel that these types tend to herd together and give each other a reason to continue with their behaviour by giving one another a few up-votes out of sympathy...
I would also say that the competitive nature of men in general (I'm a dude, and I won't assume anything on behalf of the women here) would contribute to certain sub-groups within this population with an increasing need to out-do one another. Thus a majority of the discussion devolves to one-liners and people trying to be funny, when all they do is come across as cynical while at the same time contributing little value.
In addition, attempts to be funny on a message board are somewhat akin to guessing on questions on your SAT exam: there is no penalty for trying. Whereas weakly setup arguments can carry a penalty of getting a barrage of refutations making yourself look (or perhaps more closer to the truth - FEEL) intellectually weaker.