[Miller]

Allergies, mites, skin infections and insect bites are the most common causes of dog food allergy symptoms
and intense scratching in dogs. Some types of masses, specifically mast cell tumors, can be very itchy and require immediate action.

https://www.petmd.com/dog/emergency/common-emergencies/e_dg_scratching
https://pets.webmd.com/dogs/guide/dogs-and-compulsive-scratching-licking-and-chewing#1

Hope the above links help.