My G & M do it too. But it just so happens that they are also very sloppy. I have gotten into the habit of putting a dish towel or paper towel under the food dish. Every night, when they have had their fill, the cover the food dish with the cloth or paper towel, then uncover it later when they are hungry. So, my vote is that it is a "save it for later" scratching, trying to cover it up, so that it will be SAFE for later.