In a word, static.
Digital cameras are electronic devices, and they also have moving parts, both plastic and metal. This is a great combination for build-up of static charge and for sparks to fly.
These sparks — even very, very tiny ones — can cause malfunction of the electronics or even permanent damage.
If the camera is just stored in low humidity and not powered on, a problem is less likely.
Your original list-based question implied that there's some sort of magic level, with different behavior at each threshold. That's not the case. It's just that the risk goes up as humidity goes down.