Digging can be due to anxiety, stress, or boredom.  Your dog should not be allowed in the back yard unsupervised.  Make sure enough toys are present and make sure your dog gets enough stimulation (walks, play time with you, etc.).  When you are not home your dog needs to be in a confined, safe place like a small room or a kennel.  Review your dog's environment to make sure there is nothing that has changed or is causing stress.