Sounds like a muscular tic. Sometimes they are normal, sometimes they're caused by an underlying muscle or neurological condition. Usually there's not much you can do about them, and may not be any reason to anyway. You might want to mention it to your dog's vet.

My old girl (who suffered from hypoglemic seizures during the last year of her life) used to get "little twitchies" around her scalp, eyes, and mouth when her blood sugar was crashing. When I saw that, I would give her a spoonful of Karo syrup ASAP. Most times we were able to fend off the seizure.

That doesn't sound like your scenario, if the shivers just come and go, but you might mention it the next time you are at the vet.