I don't know jack about butchering, but my guess would be along the lines of others. Perhaps the deer ran a bit too much after being shot and the muscles were completely tight when it died or possibly it was cut for eating too soon. I've heard of the same occasional problem with antelope. It would make sense that if the animal went down instantly, the muscles would not be all tensed up and much more palatable.
A few years ago, I was a little more knowledgeable in physiology, but I would suspect that if you butcher the animal before rigor mortis has released/relaxed, the muscle fibers might end up permanently locked in place and tough, similar to a permanent muscle spasm. That would make the meat hard as nails. Hanging the meat for a few days allows release of the muscle fibers and the enzymes within the muscle cells to literally dissolve the meat from inside out, self-tenderizing itself.
Just an off-the-wall guess.
