The psychological aspect is a symptom of the underlying process. It is not the cause. The easy way to tell it's so: stress had to already have set in unconsciously for you to have the first "I'm going to stutter" thought. You don't get such thought when speaking by yourself, because there is nothing to set it in motion. By the time you have your first stutter thought, it's already too late. It's natural to assume it's that first thought causes the domino effect, because we can't pinpoint to the first domino.