You may never find out. A vet visit is the best start. Here is a partial list of possible causes:

• brain trauma
• high fever
• toxin exposure
• brain tumor
• central nervous system disease
• idiopathic epilepsy… which means seizure disorder of undeterminable origin

Expect an exam and blood tests as a start, to rule out some causes, but be prepared to find no cause. There may be a cause needing immediate treatment. If seizures recur, long term medications and monitoring may be necessary.