If a dead snake reflexively attacks itself, why does it not always attack itself?
It seems that National Geographic were just as curious, in their interview in this article with James Murphy, he states the answer to your final question - that even though the snake is dead, it can still reflexively 'attack' the nearest object - in this case, the remains of its own body - as it is not attached, it is a 'foreign object' and the reflexes result in an 'attack'.