We don't know for sure, of course, but my vet says she was most likely hit by a car before the shelter caught her. She was never claimed by her owner and would have been PTS if I hadn't adopted her.
I really wish people would be more responsible with their animals - now I have the tough choice of whether to risk major surgery or not, and if she doesn't make it through the surgery I'll feel awful, but if I don't do it and it causes problems later...same thing.