that's only happened to me one time, it was during a live feeding. he'd chew and chew and occsionally bash the pinky against the side of the cage. it looked like a slow death so i feed frozen now without incident... he just slithers up and swallows it down. the white stuff is possibly brain matter.
i would imagine they're basically punching the mouse full of holes as they purposely do this to toads in the wild because of the way toads puff up as a defense mechanism.
i dont believe their fangs actually inject venom, though i could be wrong.