TWR - With respect to disks - the energy these things are running at coupled with the totally random nature of a failure makes it inevitable that they will be uncontained. The goal of the compressor area is to raise pressure but this obviously raises temperature with it. The second you introduce fire or temperature within the casing, you are raising the temperature outside of the normal operating zone placing materials beyond operating limits and failure becomes inevitable. The subsequent release of energy however is subjective and completely dependent on so many factors. As such, sometimes a disk failure event is contained and sometimes it isn't but the energies and highly random nature of a failure makes it impossible to really contain.
The Qantas failure was also not a failure of the casing exactly, it was a defect in an oil stub pipe which fed oil to the LPT area which is already running at high temperature, oil leaking into this area makes a fire and failure of some kind inevitable. The eventual domino effect release of energy is just something the casing cannot cope with, not one made of lightweight alloys anyway. Most work is done to prevent a failure ever occurring rather than to mitigate the failure. Disks are therefore designated as safety critical because they will endanger flight if they fail so failure is not something that should ever really happen. Diligent operation, preventative maintenance and also good design are the main reason we rarely see disk failures so it would be worrying if one has occurred here.
In this case, it is hard to judge from the images and accounts so far but speculating I suspect the fire was mainly precipitated by oil but it is not possible to say why or how. The passengers and captain's account of what happened during the takeoff will reveal far more about this I am sure. Personally I am just glad this aircraft did not make it into the air.