It's simply because of expression. Often when we are angry or lose our temper, we have the a sense of conviction. We in our minds know that we are right and a boundary was crossed from the other person. This initiates a certain expression without filter. Which is our natural self. If you think about when you are stuttering the most, it's when you are repressive and filtering everything. Something to remember: expression will always help you, repression will always hurt you (and your stutter)