This is a perfect example of why you NEVER stand at the side of the engine bay in line with the fan. I was at our local dirt track a few years back watching the modifieds when one of these fans let go in the pits and hit a guy in the face. The fact that it was just sitting there, and didn't do any damage to the hood/rad, is incredible. I would almost guess that it broke off when the motor fired up.
Grant