Check for gophers, rabbits or other burrowing animals. Your dog may be bored and trying to get at something at the other side of the fence. Neutering or spaying will curb some digging tendencies. So will exercise. Digging is normal.
Dogs create dens for giving birth.
Ask a veterinarian for advice. What Not To Do Don't punish your dogged digger after the fact by dragging the pet to a hole and spanking him. Sure, your dog will cower, tail tucked between his legs. But that doesn't mean the pooch connects your anger to that hole. Punishment is probably the most misused and over-used corrective.