I came across these images on imgur.
This phenomena is something I've thought about for quite a long time. I call it "guitar face", and I see others have noticed too.
I play the guitar, and I used to subscribe to a guitar magazine. When you see pictures of guitarists up on stage playing they're often making bizarre faces like this. The magazine even held a competition and recorded a video of it which included younger players getting up on stage and making faces perhaps even more extreme than this.
At first I wondered why so many guitarists make these ridiculous faces, but after giving it some thought I think I came up with the answer. I believe it's simple emulation, or perhaps to put it another way, it's a community thing.
A few guitar players somewhere started making faces when they played. Other guitar players saw it and thought maybe they should make faces too if they wanted to get up on stage like those other guitar players, and then more guitar players saw them, and more guitar players saw them, and so on, to where now young and aspiring guitar players see the players they look up to making the most ridiculous faces when they play so they try to emulate that behavior and make those faces too.
It's not that making faces like that occurs naturally when playing, it's just people emulating what other players are doing, especially if those other players are guitar players they admire.
As these photos point out, guitar face makes much more sense when you replace their guitars with slugs.