Hi there and welcome to Just answer!! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
This is caused by an interruption in the neural control and this may be due to an unstable disc in Jack's cervical region or neck. Since this stopped almost right away,. this may be minor. If this happens again, have your vet examine him.
This could also be a petite mal seizure,. This is a mild form of Epilepsy that can manifest in many ways such as this. This also resolves on its own and may never happen again. If this happens again or is worse, then your vet, if this is diagnosed, can prescribe anti seizure medication.
Good Luck! Sincerely, XXXXX XXXXX