There may be some sort of distinct smell that triggers this digging/eating, such as a ground hog or chipmunk. Have you noticed any holes or signs that animals have been in your yard? It also sounds like your dog is very energized. It is great that you take it to the dog park but it may be so crazed when it goes outside that it is tempted to eat and let out energy. I would say your dog is trying to make itself throw up, but that is only if it's consuming the grass. This sounds like it's just littering the yard. Nonetheless, I would consider looking further into what may be causing this in your yard and not so much from your dog. Even with all of the yard tearing up, aren't dogs great?