I have seen the cats in places like PetsMart do that and figured it was the only thing that they could be sort of "inside of" (we all know cats feel they absolutely must be inside something ). I don't have any thoughts as to why your cat would be doing it at home though. Maybe you could put a box of some sort near the litterbox or even a paper bag from the grocery store. That might distract them from the litterbox and maybe they might find that sort of place more appealing.