Stress on wing/fuselage-joint of aerial water tanker aircraft - and possible mitigating measures
The problem isn't the relief of stress from dumping of loads.  If it was, you would have the same failures happening because of the act of landing, where the wing goes from being heavily loaded to unloaded in about the same amount of time.  And it's once per flight in any case and is something that all airplanes have to cope with.

Fatigue is all about the number of load cycles, the beefiness of the structure designed to handle it, and identifying the "weak links in the chain" stucturally.  The problem for tankers is they spend a large amount of time being subjected to very sharp  and relatively numerous gust loads during drop operations.  Working a fire, they are flying through extreme thermals and mechanical turbulence; it's a pretty rough ride.

So the airframes get "beaten up" you might say.  Effectively it's like taking a car that could handle the odd pot hole with no problem and driving it exclusively on a road full of potholes all the time.  Sooner or later something will break on the suspension, if you didn't anticipate the effects beforehand. And in the case of the C-130s that's about the long and short of it - they didn't anticipate the effects beforehand and the airplanes were already old.

So what happens is, the more extreme operating environment means the machine is living much closer to the margins of the original structural design analysis. This raises the risk that flaws in the original structural fatigue analysis will float to the surface, when they would normally stay dormant. Structural inspection programs that were working fine when the airplanes were being used in their normal role are suddenly inadequately frequent and/or comprehensive, and cracks form that were never a problem before, aren't being inspected for, and... ooops.

Unless you want to stop using them to fight fires, the answer is pretty simple.  These are the options:


Inspect more frequently, and or more invasively using NDT procedures.
Add more meat, or redundant meat (damage tolerance) to the structure.
Both (the probable result).