Well, the reason she's doing it can be found in her name, American Pit Bull Terrier. Terrier is from "terra" for earth. All Terriers, no matter how the breed has been manipulated to do different tasks, are at heart, diggers who will follow prey into the earth. Each individual dog is different, of course, and some are more driven to dig than others.
That's not to say that you can't curb her enthusiasm for the digging. Typically, we're told not to scold a dog unless we catch it in the act. Sometimes that's just not practical, though, and an APBT - or any Terrier for that matter - is bright enough to understand what they're being reprimanded for. Take her over to her latest dig, make her look at it, point at the hole and tell her firmly that there is to be no digging. Make her sit there and watch you fill in the hole, then walk her away and get an acceptable toy and play with her. It may take a few sessions, and she may never completely stop, but you should be able to cut down on the excavations.
Just remember, though, if she senses prey underground, it's going to be difficult for her to resist going after it. That's what Terriers do!