It does sound like a seizure, some dogs have them. I'd keep a close eye on him, and if it happens again, a trip to the vet is in order, to see if they can figure out any cause for it. If it is epilepsy, it can be controlled with medication for most people and dogs.

Mini Pinschers are great little dogs - though if he's true to his breed, he has no concept that he's little, I bet!