I think it's a combination between the subconscious mind: INTERPRETING stimuli + TREATING how to respond to those stimuli under a certain rule or in a certain condition. And over time, stimuli (such as confidence, tension, pressure, and 100s of other signals/cues), thru repeated reinforcement, can lead to developing a maladaptive conditioned response (where our subconscious regulates the execution of the speech plan excessively resulting in stuttering manifestations)