Dogs normally drool to some extent, such as when anticipating treats, and dogs with pendulous, loose lips may drool more than others. However, excessive drooling can be caused from fear and anxiety or conditions such as poor dental health, stomatitis, glandular disease, heat stroke or rabies, according to WebMD.
A dog may drool excessively as a result of mouth pain due to periodontal disease or abscesses in the teeth. If a dog is acting irrationally and excessively drooling, rabies is a possibility. Salivary glands can overproduce saliva as a result of injuries such as head and neck lacerations, and these glands can develop tumors, cysts or infections that can cause increased drooling. Poisoning and tranquilizers are also known to cause increased drooling, explains WebMD.