Your state does atleast have a law stating that if a dog is chasing livestock, the owner of the livestock can shoot the dog, and seeing how you weren't with your dog and it was running at large, this may have been what happened. Of course there is still a chance that it could be a hunter but either way your dog was running and it is your job to keep them safe. Maybe your dogs will have to be tied up even in the fenced yard to help keep them safe or maybe you could find a used dog kennal they can stay in when you can't be outside with them. I hope your dog is feeling better and gets checked by the vet.