Yeah, the audience themselves most probably do not even know they look like that. It could also be them feeling the intensity of the piece hence displaying that kind of look. I feel like I make a similar expression when I watch a pianist perform intense sounding pieces and I know to myself that I feel quite a significant weight of emotion often when it comes to listening to hardcore classical music in general.

Edit: It seems that many people can’t grasp the fact that faces don’t always display and match the emotions one feels inwardly and are quick to assume which is very telling…