Cat seized during professional grooming?

From my experience, I would say its highly unlikely that the shampoo caused it, unless the cat ingest a large quantity of it. Unfortunately, for a 15 yr old cat to start having seizures usually means there is a brain tumor (as opposed to epilepsy which usually sets in at 2-5 yrs of age). Highly stressful situations can sometimes bring on seizures, if say the blood glucose level drops very low, or goes very high, but the vet would have been able to see this on bloodwork. You may want to consider seeing a veterinary neurologist if you want an actual diagnosis. But it will cost a bit to do a MRI or myelogram. Good luck.