As mentioned in an earlier answer, dogs are more than just companion animals. Every dog breed was bred for some purpose. Dogs had jobs. Therefore, these innate traits are in them, no matter how much sofa time we allow them to have. As a result, these dogs need some mental stimulation to keep them sane. Certain breeds and lines absolutely require more than others, but all require some. So, a dog is going to provide his own mental stimulation if it is not provided for him….hence hunting down all the traces and smells of moles your backyard has to offer, or just seeing how far he can dig that hole. Many dogs form the “digging habit” from digging early in their life and no one telling them not too. When these dogs arrived in their new house and yard, after everything they could possibly explore above ground had been accomplished (about 5-7 months), it is only natural they would begin going downward to continue the exploration process and satisfy their mental needs. If dogs do this for so long with no direction not to, there is a very good chance they will form a habit of digging. Of course, certain hunting breeds are much more prone to dig, but any dog will dig if they have not been shown proper structure and/or provided proper mental stimulation.