I don't think that's related to stuttering - as far as I can tell, stuttering is specifically linked to the "speech" area of the brain and writing (or even singing, in most cases) won't trigger it. That's very curious though, I hope you can find some answers! My (uninformed) guess is that it might be a motor control thing, since writing requires particularly delicate movements and so it seems logical that doing it quickly would make your nerves and muscles more likely to get overwhelmed and "jam up". I'm not sure whether there's any specific condition that causes that though. (Edit : forgot a sentence)