Turbine discs are made from steel forgings and are rigorously tested before and during the engine manufacturing process such that a catastrophic burst failure in operation would be most unlikely.
An overspeed is different. Any turbine disc will will burst is a sufficiently high rotational speed is attained.
Look at an image of the Trent engine and you can see that the biggest and consequently highest stressed disc is at the rear of the engine.
And it looks like this is the disc that failed.