I wonder if the location/soil makes a difference, too. At my old house, none of the 5 greyhounds there ever dug. The soil was hard red clay, and we were surrounded by little yappy dogs who started barking the instant either the greys or me stepped into the yard.
New location has no yappy neighbors (as of yet) and I think the greyhounds are hanging out longer in the yard. Also, the soil is looser. Both greys, who never showed signs of digging before, have excavated some big holes at one side of the house.
Reminds me of that song, "Nobody loves me, everybody hates me, think I'm gonna go eat worms. Big fat juicy ones ... "