I wonder if the voice has something to do with it. When speaking out loud you make sound using muscles in your throat. Any discomfort in your mind, such as anxiety, traumatic memory, or something unconscious, may induce some reluctance to make sound, causing the muscles to contract inappropriately. When you whisper you make all the same mouth movements but don't use the voice, just air, so the reluctance to make sound is bypassed. Just an idea, but it makes sense to me.