Koi missing Pectoral fin!

The other day when I went to my pond I was surprised to see that one of my
koi lost one of his (or hers) pectoral fins. What are some of the possible
reasons that this could happen? I don't have a raccoon or bird
problem...but now I have my two dogs "on patrol" just to keep things honest
by the pond! The dogs don't go near the fish and I never see the koi
"fighting" that would cause such a thing to happen. The fish appears to be
fine now...was having a few problems maneuvering at first but it's amazing
to see how well he adapted. Everything in the pond is in order, no fin or
tail rot disease to be seen. It just has me baffled on how this may have
happened. Any ideas? Thanks!

Bob
Long Island, NY