Repeated injuries on the same dog? How to handle in the future?

Hi all, troubling question. We watch a lot of dogs for daycare and boarding but we have never had this happen before. One of our regulars (about 5 days a month) is a great dog and we've been watching him for over a year. The owner is really nice as well. The problem is about 6 months ago he got a cut on his ear and had to have stitches. We we're watching him the whole time and have no idea how it happened. We told the owner right away, paid the vet bill and gave her some stays for free. When I say we have no idea, I mean it. My husband was watching him when it happened. There was no playing, no falling, no yelping. The owner was very understanding and he has been coming back for about 6 months with no problem. Well yesterday it happened AGAIN. and my husband and I were both outside watching this time. He went behind our shed, and came out with a cut ear. There is literally nothing behind the shed. After the last time my husband scoured the entire property looking for sharp things. Our shed isn't even metal, it's wood. He did this time too. We cannot for the life of us figure out how this has happened. We have 10 dogs a week running around the back yard and this has never happened to any other dog, even our own. Again, we paid the vet bill and gave her stays for free. But she wants to come back again! At this point we are scared to watch him, even though we love him to death. We give her a big discount so it's not really worth it to watch him if this keeps happening. And plus I don't want the poor guy to get hurt again! Any advice? Thanks! (He is an American bulldog, 3 years old)