I remember when I was a kid that I had trouble pronouncing 's', and would skip it, use some sort of 'f' instead, or just find (or make up) another word to use. That turned into regular stutter later on somehow, but I don't remember any discrete events for it. - Btw. don't worry, your brain has no agency separate from the whole that is you. We don't know quite how stutter works yet, but we definitely know it can be difficult to manage. :)