There can be a lot of reasons. Is there any diagnosis? Hypoglycemia can cause seizures, hydrocephalus (can not be diagnosed by appearance in a Chihuahua) and other neurological disorder's. They are prone to Atlantoaxial instability (spine not connected correctly). If your pup still has a fontanel or a larger one an injury to the head may also cause seizures. Liver shunt. Could just be epilepsy. Where did your pup come from?