The problem is that it was still being used despite that "in the late 1960’s the aircraft’s library showed a break in records keeping of over four years" while being used by a CIA front company (source: https://theaviationgeekclub.com/the...e-wings-fall-off-during-the-2002-fire-season/). I think it's negligent and irresponsible to be using an old cargo plane that was missing over 4 years of its records, especially for the purpose of dropping retardant which - as you point out - goes "through extreme stress" for that.Not sure the C-130 is a good comparison. That was an airtanker used on wildfires. Those aircraft go through extreme stress when dropping retardant.
If I remember correctly the cause was micro cracks that were not discovered during inspections. Cracks formed from many flight hours doing retardant drops.
If the crash was not a mechanical problem, it could have been a pilot wanting to kill themselves.
I'm not saying that the wings on this 737 snapped of for the same or similar reason, or that I know why the wings would've snapped off; I'm just saying I don't get what occurred unless the wings snapped off, because that seems like the only thing that could explain what occurred.