Orthopedic pain, dental disease, and ear infections are leading causes of sudden or escalating aggression. An animal in pain learns to bite or swat to prevent being touched. A dog with hip dysplasia may snap when you reach for its collar because the movement hurts. A cat with a tooth root abscess may hiss at its own owner during a petting session. Veterinary treatment of the underlying pain often resolves the "behavioral" problem within days.