On top of the 12 years this ball joint was in place, this a arm has had 40 years of driving heaped on it. The threads on a ball joint are very shallow. They barely look like threads at all and they are no where near as deep as a traditional SAE style threaded bolt. Multiple replacements over decades of use to simple eat up material in the a arm. There was probably enough material to thread this ball joint in, get a bite, and torque to spec, but because of the loads they carry and the pounding they take, it eventually lost its bite and pulled through.
I'd say its an a arm failure.