I'm not saying the bike is definitely a Victory Hammer, but I've got black leather saddlebags that have "Victory" stamped into them and my bike has a fat tire (see my avatar). So a Hammer is a good guess, but one of the other Victory models also has a fat tire. I think it's the Vegas, but not sure.
I don't see how the bag just fell off from a "slight bump" in the road. I mounted mine using the brackets made for the bike and the bags, and let me tell you, they are solid. There are four screws holding each bag to its bracket.
Anyway, good luck on getting the bag back to its owner. Very nice of you to do this.