A dog shaking his head so much can be caused by a number of things.
Some things that can cause a dog to shake it's head are tremors, allergies, something biting the dog, fleas and itchiness.
Some other reasons that are common for a dog to shake their head a lot are itchiness due to skin allergies. Irritants trapped in the dogs ear such as grass seeds, water or insects.
And bacterial or yeast infections and Inflammation of the dogs ear canal
The dog may also have ear mites that could be causing the dog to shake his head a lot so check for ear mites or other soreness around the ears and check for fleas or insects that could be around your dogs neck or on his or her head.
If the dog continues to shake his head longer than a day then you should take the dog to the vet to get checked out.
Sometimes health issues can cause a dog to shake his head so have the dog checked out if possible.