I think maybe it's behaviour that's instinctive - in the wild dogs would scratch around to make a bed for themselves - rough up the ground, the leafs etc - that's my thinking anyway. My last dog used to scratch up the carpet in a certain place to make a bed for herself [even though she had plenty of choices including the sofa...!]
If you click on this link - here's my foster dog Roddy - he does this to the bed every day..... I kind of think he enjoys doing it too.............
http://forums.boxerhub.com/f15/roddy-bed-2135/