If the dog is not scratching at the areas of hair loss, then the hair is probably falling out. The hair may be falling out in patches because of infections such as pyoderma (bacterial skin infection)
or dermatophytosis (fungal infection)
or because of demodicosis, a disease caused by an increase in the number of Demodex mites within the hair follicles.
Your veterinarian can diagnose these conditions through simple tests that can be done in the veterinary clinic.