Destructive behaviour in the garden such as digging is an issue I'm often called upon to resolve, with some dogs having turned a once well manicured area into a dirt bowl. It needn't be that way! Resolving digging problems takes some lateral thinking and a dose of patience to arrive at a solution.
Strange as it would seem, but I don't hear of many people's dogs burying bones in the garden and digging in the process...perhaps the giving of bones is on the decrease?
Digging is of course a natural behaviour, and some breeds such as the terriers are far more likely to indulge in such a habit. Your dog may be digging to create a cool patch, so ensure that it is adequately catered for in warmer weather.