One of my cats, Phantom, does this a lot too. With her, I almost think she just likes the feeling of the carpet (or the towel on the floor in front of the shower) better than her litter. She is getting better, though... As mentioned above, the first thing to check for is a UTI at the vet. If she's healthy, it could just be a texture issue or something to that effect... maybe she doesn't like where the litter box is located, or maybe she was using it one day and something startled her... I know sprays exist that can calm cats if its a nervous issue, and I believe there's one for keeping cats away from certain areas, too.