It could be two reasons that come to mind. Bad, dry sheds will pull out scales. This is quite common and find this will happen at least once with all my snakes. However, if this is the case, you may want to start raising your snake humidity a little more while it is shedding.

Second is live feeding. If you feed live, the best places for your snake to receive damage is around the head and neck area. The mouse/rate can bite and scratch scales off.