I think this are normal reactions of the autonomic nervous system. There is nothing wrong with it. I remember that the carp my mother cooked jumped often without head out of the roasting tray.
I have read before some time the story about a very evil pirate called Störtebeker, who was decapitated in Hamburg for his crimes. The major of Hamburg and Störtebeker made a contract, that those of the fellow pirates will be freed, that Störtebeker will pass after his execution. He seriously passed eleven, before the hangman throw the block before his feet. True story.