from what i can tell if you start winning more than 50% of evenly matched games (by their definition), they start putting you up against better players to try and bring down your win ratio to 50%. if you continue to beat these better players, you get put up. opposite would happen with going down i would imagine.
a friend and i are in a situation in 2v2 where we are in bronze league, but keep getting put up against platinum or diamond players. my friend is around that level, but it's hard for me to keep up most of the time.