If this in fact did occur........I could see it happening due to a bearing failure finnaly wearing out and letting go. At these kind of rpm's, a bearing failure can be pretty amazing. If the rotor is brittle hard it may have acted upon itself once the destruction process began. It would be like running in a blender with all those pieces acting to chew everything up. I've seen this type of failure in the field in work in. Usually in order for this type of failure to occur, the containment vessel (engine casing) has to be as hard or harder than the items inside it. Did the aluminium engine cases come apart as well? I would have expected heavy vibration to oocur prior to the event.