That makes sense. It seems the expressions are often tied to the music —raised eyebrows when playing a high note, something like surprise when hitting cymbals, scrunching when it’s nasty. So maybe involuntary but relative expressions from concentration and focus?

I was initially going to call it guitar face, but drummers make some wild expressions too. Though as a listener, I will also sometimes involuntarily make them too. Seems most of us aren’t immune.