So, how many times had it gone through overhaul and repair where such a massive propagation had gone undetected even as the fracture surface may have been discolored by cleaning agents etc.
Like engine rotor disks the safety of flight is provided by robust overhaul cleaning and NDI to catch cracks before they propagate to complete fracture. It would be very rare to have an operational annomaly that would result in fatigue propagation between overhaul periods on such a critical part.