My cat Mandy has had seizures.
We didn't do extensive tests to find out the cause, but when she first had them, she had a bad ear infection, and the vet thought that could have contributed to it. Or, sometimes problems with various organs (she also has bad kidneys) can contribute. They can also be caused by brain tumors, but there's not much that can be done about that in most cases.
The first time I saw one, I didn't even recognize it as a seizure, as she was lying on the floor, and started 'reaching out' with her paw. That was it. She's had some nastier ones, though, that have her flayling around (two of those that I've seen, three days apart, back in March, but none at all since then).
Since she hasn't had any since March, the vet says just to keep an eye on her.