Hey, my cats used to do that with their clumping cat litter and now I know why! One day I came home after buying a bag of cat litter. I was being lazy and just put the bag of litter by the door on the linoleum. The cat somehow ripped a hole in the bag and litter spilled all over the floor. He proceeded to play in the litter until I came to see what all the commotion was. That was when I realized why they like to play in clumping cat litter. It is quite staticky so in a plastic litterbox (or on the linoleum) the litter tends to jump due to static. That's why they pounce on it like something is moving. See the cat isn't crazy, something actually is moving. You have to get down to cat level to see the jumping cat litter. Then you will realize why your cat is so interested in it. The fun subsided after my cats got older. Maybe the jumping cat litter gets boring after awhile.