Stressful scenarios and emotions + heightened anxiety cause a trigger. Stuttering is usually caused by a neurological pathway so your brain is reflecting a "wrong" order to your motor neurones that deliver the action of speaking and since it happens again and again the brain interprets this as the right action - that's why practicing smooth speech techniques is the most effective form of treatment bc ur retraining ur brain to create a "right" order (doesn't always work but that's how i've been explained it in the past), when we are in heightened situations of anxiety we anticipate a lot of our stuttering so our brain is already forming that anticipation and acting on it