but I am certain W&B wasn't the cause.
And I am as certain as it is possible to be that it was.
When the original FU24's were being used for topdressing, the pilot had a single seat in the cockpit, the loader driver had a bench seat behind the hopper, the hopper was actually your back rest. If the pilot had for some reason upset the driver, then on the flight home it was possible to get even by crawling down the back of the fuselage, thus putting the C of G a lot further aft, the pilot had a moment of "what the F***k, and then figuring out what was the problem move the control column back and forward very rapidly. The driver rapidly resumed his proper seat.
As you can see in the photo you printed, the load is a lot further aft then when the aircraft was fitted with a hopper, the load then being just about right on the C of G, and not capable of being moved.