Without Prejudice
Very disturbing pictures of the tragedy. "Eye-witness" reports always see the explosion or flames first because that is what draws their attention. Having watched the video a few times now, the Herc appears to have been pulling out of a descent when both wings separated, almost simultaneously, within the centre box section. The flash fire clearly follows the separation.
There is no spar, in the traditional sense, on the Herc wing.
Since her first flight in 1955, there have been many wing failures due to overstress - Cb penetration is a favourite. The UK MOD learnt, at considerable cost, the importance of using beefed-up centre sections when operating the 130 on frequent low-level ops. Contrary to Lockheed's advice, the MOD opted for "standard" wings, in order to save (I believe) £7k per production airframe on the initial order of 66 aircraft.
When, in the '70s, the test rig at Marshall's of Cambridge unexpectedly broke the C130 test box, the entire RAF fleet was grounded temporarily whilst the NDT team travelled the world to clear each overseas located aircraft back to base. The entire fleet's centre sections were duly replaced at around £250k each.
The military Herc is cleared to pull 3g, clean, operationally, and has a G-meter on the panel. I cannot vouch for the civil version, nor do I know which category the downed firefighter came from. Repeated ops at low level will eventually crack the centre box and result in failure at less than the optimum stress limits.
777AV8R is right about negative g, but all large transports share that weakness for obvious reasons. However, wings rarely fail in negative g but on (the) subsequent positive.
My sincere condolences to the families and friends of the crew who were conducting a dangerous and valuable public service, should they happen to read this thread.