Well, stress increases the difficulty of everything, right? You could be neurologically predisposed to stutter, but you manage it better when you're clear-headed. Then, under stress and discomfort it gets harder and you stutter more often. I agree that it's strange that stress has such a sharply negative effect on stuttering. That mechanism needs to be stuttered better. During my research I read that the damage that causes stuttering is located close to the source of Tourette's; Tourette's is also exacerbated by stress/discomfort. So, not a clear answer, but it's interesting and I hope we get more data.