If a wing is providing a lift force to carry a certain weight, and that weight is removed; the stress and strain in the wing structure will be reduced. Like the string on Hans Brinker's balloon: the tension in the string, that was holding the weight up, suddenly reduces to near zero when the string is cut.
I suspect that wing failure in accidents of this nature is actually caused by a rapid unload, then re-loading of the wing, as the payload is dumped, followed by a max rate pull up. The fin of that A300 was snapped off by a rapid load - unload - load sequence applied by one of the pilots.
If you have an aircraft such as a water bomber or a crop sprayer that is subjected to rapid pull ups fairly often - in a series of alternately maximum and zero payloads, i.e loading / unloading / loading; and this is coupled with ageing and possible corrosion of the internal structure - which might not be inspected or spotted during inspections - you might have a recipe for serious structural failure?
PS; physicus, on planet Earth, the weight of an object is the attraction force produced by the action of the Earth's gravity on the mass of that object.