Hi Bob, I think several things are at play.
As one said, snakes are hardwired to secure prey, then determine what it is after. Seen that many many times in nature.
Also, they learn where food comes from and WHERE it comes from.
Combine that with the grap the biggest part behavior and you have them eating your hand.
Theres more, in group cages, its the monkey see, monkey do, type of thing. They will watch another feed and then repeat it, and often repeat it poorly, hahahahahahahaha
Lastly, we tend to lose track of where are animals are. That is, they are hardwired(instint) for nature and they are not in nature now. We tend to think what they do in captivity is normal, but in most cases, there has to be insanity(not Sean T) Remember, insane is to act different then normal, its not necessarily bad. Just different. Captivity does support different behavior.
All in all, my bet its a weird learned behavior and its normally rewarded, after the snake does that, it is given a mouse(food item) which trains the animal to do it again. Reptiles are good with reward learning. They also attempt to improve on the previous event. Cheers