Why does a snake move after you cut off its head?

The same reason chickens run around or other animals squirm or kick after decapitation. Most animals do this. It is a result of the total loss of neural input from the brain to control muscles. The brain prevents muscles from moving as much as it controls their movement (all muscles work in pairs, when one muscle tenses, another must relax). Also, whatever stored up electrical energy is left in the body is left uncontrolled by the brain, and just travels unimpeded through the various body parts trying to get to ground, which causes more muscle spasms.