There isn't one! Alot is being made of the fact that he is shot once from the front, and once from the back. If you look at these two shots though: Galvatron's feet are facing in the direction that the shot is coming from in both of them. Therefore both shots are coming from Lockdown, who is stood by his ship. Prime is just facing in a different way each time. So I assume what happens is: Lockdown shoots him from behind, by surprise. Prime turns around to to face lockdown while starting to get up angrily, putting his mask back up (the shots we've seen loads of times), but is then hit square on in the chest and flies back onto markymark's car.