Old Age-if your hound is getting up there in age he or she may be more prone to disorders that cause trembling or cognitive deterioration. Some medicines like Deramaxx, Rimadyl or Dasuquin may help. Consult your vet for more info.
Disease- if your dog has renal failure or kidney disease he may drink and urinate more and your dog may also shake. Treatments like furosemide may help. Another disease that has symptoms such as shaking is Addison's disease.
Poisoning- any harmful ingredients given or consumed in high doses can cause uncontrollable shaking. If you think your dog as gotten into something it shouldn't have contact the ASPCA Animal Poison Control Center immediately at (888) 426-4435 and call your vet.
Stress-InducedTrembling-thunderstorms,severe anxiety, a trip to the vet or an unfamiliar place an cause shaking. Your vet might also recommend medication like clomicalm.
Happy Shaking- finally a good reason your dog can shake uncontrollably is they are happy and excited to see you, do something or go somewhere.