Are you certain it is your dog? Have you actually seen her doing it?
I say this because it may be a fox or something else getting in your garden.
If you are certain it is your dog it may be worth getting her checked out by the vet, there may be a medical reason - eg she may be loosing feeling or control of her back end.
Any change in habits should really be checked out by a vet to make sure it is nothing serious, remember if things are caught early they can often be dealt with faster and better.