All dogs drool to some degree, but excessive drooling (hyper salivation, is most likely triggered by events such as fear, apprehension, and nervous anxiety, as well as anticipation of food treats.
Drooling is also caused by periodontal disease, abscessed teeth, and stomatitis.
Motion sickness is a common cause of drooling.
Other causes of drooling:Rabies
Heat StrokeDistemper