Maybe I'm missing something and someone can help me out, but:
In Toy Story, it's established that the toys can move and speak in the presence of humans, they just don't. After all, Andy and the misfit toys scare Sid off by talking to him and moving in his presence. So when they flop down when Andy bursts into the room, that's a choice, not something out of their control.
But for two-thirds of the film, Buzz Lightyear doesn't think he is a toy. So why does he become immobile in Andy's presence.