I don't know the exact science behind it, but I've developed twitches from pushing blocks my whole life. I started stuttering when I was 5 and the twitches began at around 12. I'm currently 21, still have them. The twitching is called "secondary action", which I'm sure there is lots of research on because it occurs for people with all types of disorders. The twitches occur WHILE stuttering - because your body is trying to force out a sound, you might start jerking your head or hand as a reaction to that force. I've heard of all different types of twitches. They key is to just breathe and be calm, which I know is NOT always easy. Im still working on it myself!