Lost our first hen

We got our hens in April, had 10 of them, now 9. We've got a chicken tractor that my dad built and we open the door in the morning and close it at night because we live in the country and they have lots of land to run around on. Turns out that the daytime hasn't been a problem for us. Friday morning when I went to let them out, I discovered the door was already open and lots of chicken feathers were in the run. Most of the chickens were out (or gone, I didn't know then) and there were two sitting in the nest boxes acting like nothing had happened. I had no way of knowing how many I had left until that evening. Turns out, there were all still there, and I didn't notice immediately any signs of damage, which I thought was weird, so I thought maybe it was a fluke that the door got open, and maybe my chickens had fought each other. That was until this morning when my husband discovered one of the hens wasn't moving and had a gash in her back. Flies were swarming around her. By late this afternoon, she smelled to high heaven. We didn't know what to do. We're new to the whole chicken thing. My neighbor came down and put her out of her misery, which about made me lose it (I'm 33 weeks pregnant, hormonal and very sensitive sense of smell). He thought that a raccoon did it since the door had been opened, but thought it peculiar that the raccoon didn't take the hen. My father thinks that the rooster accidentally got her with his spur. Would that explain the feathers? Maybe, I don't know, but it wouldn't explain the open door. I feel awful, I wonder if there was anything we could have done for her had we discovered her injury sooner. Thoughts? And thanks for reading this long winded post!