I wear a mask every time I'm around folks in public (just not on my daily, solo walks), but I'm fortunate in that I have been able to work from home this whole time and don't have to wear one all day. I've found that when I do wear one, I tend to unconsciously clench my jaw while wearing it. After even a short (20-30 minute) trip to the grocery store, I can feel the tightness in my jaw and neck that results from it.
Maybe that's what's causing, or contributing to, headaches for some of the Cast Members. I'm sure it's not a stress-free job in the first place, and under these circumstances, it's got to be a lot more stressful.