Brain Tumors. A diagnostic test, such as an MRI or CAT scan, can determine whether or not your dog has an abnormal growth on the brain. Brain tumors, especially those in the cerebrum, can be a common reason for seizures in older dogs, but they are rarely the cause of seizures in younger dogs. The highest incidence is found among the short-nosed breeds such as the Pug, Boxer, Bulldog, and Boston Terrier. Tick-induced Diseases.