One of the most common causes of alopecia is mange, which caused by the mite Demodex.
Hair loss can also occurs when there is a disruption in the growth of hair follicles, often from infection, trauma, an immune disease, or endocrine system abnormalities.
If there are multiple missing patches of hair, it could be associated with an inflammation of the hair follicle. A more widespread area of hair loss, meanwhile, may indicate a more specific disease pattern.
Read more at PetMD: Hair Loss in Dogs