Without an autopsy, you may never know what happened. It does sound like a seizure - the problem is figuring out if the seizure directly contributed to his death, or if it was an underlying condition that caused both the seizure and death. It could have been a stroke, or even a heart problem. Any time oxygen is reduced to the brain, seizures become possible. Any chance he could have developed heart worms?
He did not have heartworms or any illness. He was a fit dog with no problems.
My guess (and this is only a guess) would be massive heart attack or stroke. Either would stop blood flow to the brain and could cause the seizure-type symptoms. I've seen people have strokes and heart attacks and it sounds very similar to what your dog experienced.
My very sincere condolences to you.
You might research " DCM" - dilated cardiopmyopathy. In many cases this results in otherwise healthy dogs "suddenly dropping dead' and 7 yrs. of age is typical. We lost 2 dobies to it and are treating a 3rd now.
My cat did the same thing...we did not have an autopsy, however, the Vet did say possible cardiomyopathy...I doubt the seizure caused him to die, it was the condition that caused a sudden seizure and then death...I know the vet said that my cat did not suffer, and I am hoping that you can take some solace in that. I know it is so hard to lose our friends...I am sorry for your loss and wish you peace.