How often is he fed? IME feeding the snake in it's enclosure does not cause any aggression spot clean their tub. However, if the snake is hungry and used to being fed live prey, than they may instinctively respond to the heat signature of your hand by striking.
Husbandry could also be the issue. Does your BP have multiple hides in his enclosure so that he can feel secure? What's your set up like (substrate, temps, etc)?