Large fault in a building which eventually tripped the main, before it tripped it cooked the ground wire.
if this were to happen, I would say there was something wrong with the main bond at the service. A grounding electrode system protects from external sources like failed transformers and near by ligthning strikes. The voltages are much higher so the earth's resistance is easily overcomed. If a fault as you described was cooking the the GES, that tells me the main bond is not doing what it is suppose to do.
