@Craven de Kere,
6.) Wave effect. (I.e., non-lasting popularity.)
6a.) People get annoyed that something has been voted down, say so, and get a bunch of people who agree with them (resulting in a big swing); and vice versa (people think it's stupid that something has so many votes, and start down-voting it).
6b.) Popular threads that started strong and then devolve into retreads or sniping or just go on too long and people get tired of seeing it continue to pop up in "New Posts."
Related, but its own thing:
7.) Contrarianism. I think people will vote down threads that seem to be trying
to be popular, and will vote up threads that are more fatalistic (i.e. that the original poster assumes it WON'T be popular). (I think contrarianism will come into play in other ways too, that's just one example. People hate feeling manipulated, or told what to do or think.)