Yes the brain has developed pathways for speaking, which it follows. It so happens that the brain uses a different speaking techniques, style and during all that, a different breathing pattern. Stuttering is neurological, but mostly also mental. Trust me, I've gone through a lot. Your lack of proper speaking techniques/trauma have led to your brain taking an incorrect pathway for speaking, which results in poor mind-muscle connection and poor breathing. Best example: Stutterers often feel suffocation during speaking, including me, providing evidence for my point. Now, the reason people usually advice to stutters to speak slowly is the fact that the breathing slows down, you are not in a hurry, and hence you take in good amounts of air, effectively reducing stutter. Now, stutterers have a habit of over exaggerating their muscles, and poor breath use when speaking, which leads to their stuttering. Now, when you speak loud, it is probable that the brain had developed an already good speaking pattern, and hence better breath. I am pretty sure OP didn't feel suppocation during his loud speaking.