Why are my cats diggin?

You might have moles or some similar small animal in your garden.

Either that or you cats just like digging, I've only met one cat in my whole life that liked to dig but its not impossible.

To stop the digging of your garden first make sure there are moles or what not. Then make your cats their own sand box, a bit deeper than a kid's sand box, and fill it with potting soil. Then bury some kitty treats while your cats watch. You'll also prolly need to use some kind of repelant on the garden to keep them from coming back to it.

They have a very keen sense of smell or hear, maybe they hear an insect stirring about under the dirt.

Looking for small critters...they hear something in the ground.

They're looking for buried treasure

chipmunks