I hear several clues.

If any of the fish food is "gray" it's likely moldy. Typically, in a bag of moldy fish food, you'll see a variety of color. You'll see some pellets gray, some are normal color, some are faded. In a normal bag of fish food the color is consistent. Moldy feed has aflatoxin, which is fatal to fish. When moisture hits feed, it molds.

Second, when your pond drew down, all that left was water. If the level dropped more than two feet, then around 1/3-1/2 the actual volume of water evaporated over months of drought. As that happened, concentrations of minerals and metals and nutrients increased. Since that happened slowly, your fish adjusted. When the rain hit, it diluted months of dissolved stuff. That can add to the issues by quickly changing osmoregulation.

Third, if you had a plankton bloom, which is likely since you are feeding the fish, a fresh heavy rain can immediately kill enough of the bloom to cause rapid water degradation and oxygen depletion.

Fourth, almost three years into a catfish feeding program, your pond is beginning to reach its capacity for fish.

When you combine an expanding standing crop, moldy fish food, a drought and rain, you have the set-up for a perfect storm to kill fish.

Teach a man to grow fish...
He can teach to catch fish...