"If Buzz Lightyear Doesn't Think He's A Toy, Why Does He Freeze Up When Humans Are Around In Toy Story (1995)?"
"Because he's on an alien planet and that's what everyone else does when these giants walk into the room. He needs to blend in.
It also might just be an instinct all toys have. You can't exactly have toys not holding still just because they don't consciously know any better."