There are a ton of possible reasons, and we can’t tell you which it is unfortunately. Anything from antihistamine use to diabetes to dehydration to oral thrush to being a smoker to certain autoimmune diseases to just sleeping in a super dry room could be causing it.
The fix might be to drink more water, get a humidifier, or see a doctor. Or something else!
I would start by trying to assess if there are any commonalities on the days that precede the nights when you wake up with a dry mouth and work from there!