There are various reasons, the vet said it could be from pain, low blood sugar, low quality food, and other unknown reasons. All of my pups have experienced head tremors at least once. Brady had them when he broke his toe, presumably because of the pain. He had the tremors at the vets office, mostly while being examined when the vet was manipulating his toe, vet wasn't concerned at all.
Abbie had them the end of not this past summer but the one before that. I called the vet (after hours, we don't have emergency clinics) and he said it happens quite frequently that time of the year and they don't know why. He said to keep an eye on her and if she went into a full seizure, keep her safe and stay calm during it. Fortunately it didn't progress past head tremors and lasted for a short time like he said. She hasn't had one since that I am aware of.
Tucker, my Mom's dog had one the end of this summer. Tossie had one everyonce in awhile, but a "cookie" usually solved the problem.
They are very scary the first few times you see it but eventually you can hold enough composure not to freak oout...