Dogmatic got it right. The board is a microcosm of society, except that on the board there is anonimity. No one really has to be personally accountable for their comments, so there is a tendancy to perhaps be a little more confrontational or a little more aggressive than they might be in person. Additionally, some people are just more cofrontational than others, and of course, some people are flying false flags and just trying to promote an agenda, and some people, unfortunately, are just assholes. (We know who they are)
Good manners and civility in interpersonal relationships flourished in the days when everyone carried weapons on their person all the time. During those times if you spoke or behaved in a rude manner, you might have to pay with your life, or at least a good beating. Since that is an unlikely outcome from a board post, some people are emboldened to misbehave for a multitude of reasons. That is why people in large groups often do really supid things--they have some anonimity by being in the large group.
The board is a very cool thing in my opinion, because it gives a "no consequence" forum for the exchange of ideas and information. However, I think everyone should take any comments or information with a grain of salt because none of the authors are accountable, or even findable in some cases.