Why does my dog dig in the furniture?

Most dogs were originally bred to service a function.  Some dogs were bred to chase small animals, for hunting rabbits or fox.  Many of these breeds were also intended to dig for their kills, so your dog may burrow because it's born to do that.  Said differently, some terrier breeds were meant to hunt rats, i.e. the rat terrier.  These dogs will dig even if there is nothing to dig for simply because it's what they were bred to do.  Other dogs dig to create nesting areas, to warm up their area before they lay down, or to check for bugs.