According to petMD, excessive salivation in dogs can come from ptyalism or pseudoptyalism, two similar conditions with very different causes. Ptyalism is merely the excessive flow of saliva, while pseudoptyalism is the spillage of saliva trapped in the mouth.
According to WebMD, most forms of excessive drooling are harmless and are commonly found in larger breeds that don't have tight lips. In other cases, dogs drool excessively due to a tumor, cyst or injury to their salivary glands. The salivary glands are stimulated by the brain stem, petMD explains, so it's important to rule out potential brain damage if a dog drools excessively after an injury.