There are also some pike in the dupah as well... as most bass fisherman know, predator fish are not shy about hitting something that is incocievably too large to eat. A 30" pike could have wounded the fish, or a heron may have missed him.
I hooked into a smallie on the fox once that i thought swam into a snag. Suddenly the fish grew about 10lbs for a run or two. and when i finally pulled in the 10" smallie, he ahd a nice gash across the top of his back and through the "fillet-(not that i would fillet a bass!)" portion of his side. I'm almost positive it was either a muskie or a flathead catfish that hit my struggling bass...
Having a fish tank, and experience with large, agressive fish, I know fish have an amazing healing capability and a fish may survive an injury like you described. Hey, you caught it so it was likely trying to feed right?!