It may be related to three things.
Incorrect sleeping position, such as when sleeping on the stomach, saliva secretion in the mouth cannot stimulate the pharyngeal cavity to cause the swallowing reflex, thus drooling.
Oropharyngeal and facial muscle disorders triggered by facial palsy or stroke, which in turn lead to drooling.
Misaligned teeth, misaligned bites and missing teeth can also cause drooling.