My pup is quite the archaeologist. He has found the old owners buried cat (ewww, we had to clean that up), various planters that were BURIED into the ground, tennis balls that were buried (by previous owners dog), and just last weekend he found barrel rings from a half barrel the previous owner used for planting plants in. He even found me a dino! Okay...so it was a toy but it was still a dinosaur!
Maybe there's something interesting by that root or she's bored? Gunther digs even when we're out there playing with him, he also likes to stick his toys in the dug up hole.
OH! And his digging escalated when to where it is now when we started to see mole hills. No more moles but I think that was kind of a big trigger for him. I'd say though possible boredom or interesting stuff could be it.