-Attention-seeking behavior. Everyone does it to a certain extent, but some people constantly do things like making statements that lead people to ask about them. I love to leave these people hanging. They expect me to ask about some medical condition they just faintly described. I just say "Oh, hmmm" and walk away.
-Contrarians that feel like they must argue the opposite stance from everyone no matter what. Probably a little bit of attention-seeking behavior too.