Have that ( or something like that ) here, one of my oriental lilies is starting to lean due to the digging, and quite a few bushes have holes around them. Not sure in your case but after some intense observation, I found birds to be the culprits. Bug hunting I guess. Easy fix though, since the digging is happening from the TOP down, I just placed some hardware cloth a good distance around the affected plants ( about a foot will do ) starting real close to the base of the plant.
p.s.
This also works on rabbits, cats ,dogs and yes even grand kids.;-)
