This happens to me too. I will admit, my stutter is often severe, but it seems to always be at the upper end of the severity spectrum when I talk to someone who either stutters or knows someone close who stutters and is familiar with it. My guess for why this happens would probably be, I feel like I should be more comfortable with this person and be more at ease. This might put subconscious pressure on me to not think about my speech as much and just talk, which im guessing paradoxically actually puts more emphasis on my speech and its just out of control because I now have an extra focus on *not* worrying about it.