I don't remember the exact details, but the NTSB report on the crash of the FDX -11 at Newark concluded that, although the aircraft was correctly certificated to 1970s regs, the wing strength where the spar and undercarriage meet was pretty marginal.
Basically it takes very little to exceed the design strength of the wing at the root with a high sink rate at touchdown, especially if the touchdown is on one side.
So any mis-handling or weather induced sink can cause the wing to fail, causing the aircraft to turn turtle, a la Martinair/Fedex/Mandarin.
Perhaps someone can find the NTSB docket?