My 6 year old does it and use to do it as a puppy. I have no idea why, but I just let him. He always seems to do it right after I make his bed for him- I get the cushion nice and fluffy, lay the blanket in all proper.. then once he sees that he jumps right in and digs away!!
I just let him carry on- then I go behind him and fix it again when he is done LOL.
