I find it's usually the team play. One side finds a way that works and keeps at it, while the other team keeps randomly running all over the map. As a support infantry this really sucks, cuz it usually means I die first.
Mind you, I've also seen games where one team gets stomped, then next round tries something different and stomps the other, so the first team comes up with another play style and stomps again, and so on and so on. They usually are the funnest games, but kind of crappy for the multi.