[venk12]

Even drummers using their four limbs tend to make weird faces or open their jaws wide while playing.

It should have something to do with dexterity, coordination and limb independence I believe. I can still play simple beats without making faces. But I wouldn't be able to play complex polyrhythms without making faces.

[jack_codes]

When I play guitar or mandolin my mouth often moves in some sort of way as if keeping time. I recorded a video one time and noticed it. Now I'm a little self-conscious about it but it's a bit relieving to hear it's pretty common / innate.

[dist-epoch]

Maybe routing commands just to the limbs becomes too complicated and the commands spill over to related routes.

Or maybe the brain is too busy with the limbs and relinquishes face control which starts acting on random signal flows.

You can see this in other situations, for example when you are in a high impact situation you stop caring about "how you look".

[quickthrower2]

Or lift heavy weights.