My educated guess: The landing gear wing spar could have had microscopic fractures due to a combination of heavy landings and metal fatigue and could have just given way as the plane was being loaded with fuel.
In my experience with Boeing A/C, a wing spar or wing spar cap that had cracks would leak fuel. I doubt that other than ignored this is a factor. A microrosopic fracture in a an aluminum wing spar grows with load and aluminunim failures are allmost allways in a wing dectected by leaks much prior to failure... There is more structure to maintain the the intregrity of the wing to prevent failure even if the spar is cracked enough to leak fuel.