I tend to drool in my sleep beacuse of nasal polyps which i have had all my life. They cause me to breathe through my mouth.
Maybe its because you've started breathong through your mouth for some reason?
Prior to tonsil removal, I always had larger than average tonsils. In my youth, I used to drool a-lot whilst sleeping. Finally stopped and reappeared later in life.
I associated it with being exhausted (along with snoring).
A year prior to tonsil removal, began snoring at night frequently and drooling on occasion. Since removal, I've done neither.