If a defense mechanism is a response to stress or perceived threat, then anything could set that off depending on the person. Fear of a type of person, fear of being misunderstood, doubt, distraction, the list could be endless. I would argue that there is something going on between the defense mechanism and the phenomenon of stuttering which doesn't occur in non-stutterers. Whether it's neurological, or some part of the speech production system that malfunctions when cortisol increases, or a psychological block, or a combination of things. Either way, figuring out how to reduce anxiety and stress in response to all sorts of situations will surely help to allow a naturally easier flow of speech.