I don't know why Morty but I will add my speculation to what others have suggested here.

1. Some may just want to see the effect. I guess you could liken it to an arsonist lighting a fire and sitting back watching the results and lauging at the fire fighters trying to put it out.

2. Some may have a grudge or feel jealous of someone or something and see it as a means of getting even. The grudge/ jealousy may of course be at the forum as a whole.

3. I guess that it is not impossible for someone to start out thinking they are on some form of crusade to save the forum from BS but I think if genuinely does happen, they wind up rapidily losing the plot.

Jon