Its been 20 years since I worked for RR and in the jet engine industry (RR RB211-E4 era) in general and my memory is failing - but the turbine disk fragment looks like a 120 degree segment which if I recall correctly tends to suggests a high speed/energy/overspeed failure. So maybe a compressor/fan blade failure triggered a domino effect that ended in a shaft failure and resulting turbine overspeed once the load from the compressor is lost. Also the fact that the disk has no blades or even any remnants of blades might suggest they were shed with the engine running in overspeed vs mechanical damage. Either that or a straight disk failure due to disk metallurgical problem.
Obviously all of this speculation at this point but having the vast majority of the pieces will allow the RR failures (!) lab (I worked there for a summer) to diagnose stress fracture, fatigue failure etc. This is why the authorities are asking for help to find the other disk fragment to complete the picture.