Why would you be sticking your hand inside the snake cage during feeding time? You should either drop it in or use tongs. Your hand should never be associated with feeding. That doesn't make sense. Rodents are what triggers the feeding response. Not a giant human hand waving around.
The only reasons for biting is
1. You smell like a mouse and the snake is hungry
2. You pissed the snake off
3. The snake is scared and bit out of defense.