It could be breed specific. Some dogs will shed year around heavily. Keeping them inside all the time helps keep them shedding continuously. I have one like that now.

If the dog is itching or scratching in addition to suddenly losing large amounts of hair, then it could be related to flea allergies, other allergies, or hypothyroidism.

If the skin does not appear to be inflammed, bug bitten, or scaley then it's most like breed specific. When in doubt have a Vet appraise the dog.