It's human nature. The board is not much different from any other part of society. You get a few people instigating, next thing you know you have Baltimore...metaphorically. You get enough riots...people move if they can. In the case of the board, some move to other sites...some stick to Friends Only. Either way the main message board sees a reduction and as the more positive posters move on, the overall tone changes...which discourages the optimists from posting to avoid being "Trib'd."
We often laugh at the comment section of the Trib, but the same type of thing happens here as well...just inside the board rules and with more coherence.