Its common among most stutterers (like myself). I once stuttered when i thought someone was behind me as i was showing them something, i turned back and there was no one there. Its excessive self monitoring, a behavioral pattern that starts small when we're children but snowballs as we grow up. There's books out there on how to train yourself out of that. I have the nagging suspicion that the problem is deeper than that, there's more than likely trauma involved and that part has to be dealt with too