Catbird, I am guessing like others, that you had low DO and High water temps. Plus, catfish over 10 pounds in an unfed pond are simply pushing the limits we all do...old age. My guess is the ones dead, were all original stockers. It is entirely possible something sick found it's way into your pond and the CC scavenged on it and picked up a nasty bug, especially since you say no turtles or other CC scavenged any, except the recently dead one.
It is also a catch 22, when low DO causes a few fish to die, their bodies often don't float and the bacteria of decay consume a lot more DO, making the low DO even worse.
