Why is she always fiddling with the volume knob.

Yea I always noticed it and distracts me. It could be a nervous tick, like when I'm nervous I fix my glasses when they're on rjght. Although it's most likely that she's just fixing the buds in her ears to listen better.

Every artist has that issue but I’ve not seen them fiddling around with it so often.
Today I caught up and watched her GMA performance and I think I cracked the code! Take a look towards the end when she hits an off note. She gets a confused look on her face for a split second and then she goes right for the knob and gives it a big twist. From there on she’s in perfect pitch. Therefore, that must me the Autotune control.
Haha.
She actually sang that really well!

It is a bit distracting. She's like the only artist I know who obsessively does this. I chalk it up to being a nervous tic. I don't even think she realizes she's doing it.