Well there is no cut and dry reason for that. I can only tell you why it happens to me. So i have the same because i expect to stutter when i speak. I feel uneasy and like every word will be a bad one. I focussed on the bad words more then the good ones. Its the stress that you get when you feel like you need to speak perfectly and with ease. Which is not easy at all. When i am alone i speak in a diffirent way to. More quitely. There is also no pressure to speak or someone listening. That way the stress and anxiety factors are gone. This is all ofc how i experience it. Maybe ya can see yourself in something.