Ventus,it cracked from the edge of the hole, probably started at a tiny nick in the edge or a tiny manufacturing defect.
The problem with all this life factoring is that the fatigue life is determined not just by the load the joint has carried, but by the duration of the load - in effect the "stress strain history" of the joint for its entire life. Today digital recorders give us the possibility of actually recording something like that history but mostly we have to make approximations of what the loading history would look like and then test to destruction.