I been thinking about this a lot recently. I have always clenched my jaw and in turn, this makes my teeth grind slightly. I'm not sure if it's a stim, but i do it without thinking, I have tried to stop, but it's hard to stop something when you don't even realise you're doing it.
Anywho, I think i know 'why' i do it. I feel like i have to do in order to stay in control. For example, placing an egg into a pan - i have to clench my jaw, otherwise i feel i would drop it or place it in the pan too carelessly that it will break. Or when I am walking because if i don't I might fall over. Every action I take I must clench my jaw or i will lose control of my motor functions.
It sounds stupid when i think about it. As if i could actually lose balance and fall over while walking lol I don't think I would, but an inner part of me believes it so much lol