Some levels are subject to random explosions shortly after the level starts, and some bricks can fall on their own before you've ever thrown a bird. These events occur rarely but by the time you've three-starred every level in every game you'll probably have seen it a few times, as I have.
I would expect that this occurs because of timing issues, possibly related to threading. I'm quite sure that if you did a test where you loaded every level a dozen times and waited thirty seconds you'd end up with a non-zero score at least once. In light of Pubby's answer I can only assume that the Angry Birds app does not always provide the same input to the Box2D functions at the start of a level.