It has to do with anxiety and stress. The whole purpose of meditation is to shift u into parasympathetic nervous system which means rest and digest mode. But when u become anxious ur whole body turn into sympathetic nervous system which is fight or flight mode. But when u really practice meditation even if u are anxious u will still be in rest and digest mode. To obtain that ability u have to practice really hard. I obtained it that is why I am completely free from stuttering.