Perhaps it's a self consciousness thing. When one is not self conscious of their stutter because another emotion takes over, they have a tendency to stutter less. I have been in similar situations where I had to stop large, muscly, loud, sometimes drunk, and belligerent men from beating up or angrily yelling my friends. During those moments everything felt extremely clear and I somehow knew the right things to say without stuttering. I guess it was probably a combination of fear, adrenaline, and my sense of duty to protect people from harm. I also become fluent whenever I play a heated airsoft match or any intense sport. So yeah...probably adrenaline.