+1 to Damien, just a few more thoughts.
If you notice it took several whacks of the head to get the bite reflex to trigger. I know that many animals have reflexes that will suppress other reflexes so that fluid motions can be achieved (righting reflexes are examples).
I don't know if anyone has studied the species enough to know this definitely, but I bet that snakes behaviorally/reflexively doge their own tail so that they don't keep hitting themselves in the head. After the head is severed, higher level restrictions are lost, fight paths are probably lose inhibition, and the reflex to bite anything hitting you in the mouth takes over. That said it probably had to hit the mouth just right to get this to trigger.
I'm looking for some definitive information on bite reflexes in snakes, and will edit if I find anything relative. I have experience from studying amphibian reflexes years ago, and I have feeling this general trend will hold true for snakes (but I'm looking for someone who has actually studied it).