Butt munching is one of the leading causes of regurgitation. This can be a very serious condition. As the prey item works its way into the snakes digestive system the “butt” starts to digest first while the bulk of the animal is still facing toward the snakes mouth. Due to reflex actions in the prey the animal starts to move away from the digestive fluids until it gets worked up into the throat at which point the snake gags and regurgitates.
If you see that this is happening, you should pick it up and turn it end for end in his cage. This will usually solve the problem.