Some of them seemed to be playing randomly. Some of them decided that they didn't like the game (too hard to understand, they weren't getting enough candy, whatever) and cooperated in spite of partner defection as a way of checking out of the game. One guy didn't even want to know what his partner had done last time during the iteration, he just defected every time - I guess that could be called a strategy, especially since he wound up with a randomly-playing partner that time.