I think Richard is right regarding previous damage. The Cent clutch is a three plate development of the twin plate Cromwell and Comet clutch with many parts being the same. It was not known as a weak point.
Indeed, the usual failure with an overspeed is the engine big ends. If you allow Cent to run away down hill in low gear without disengaging the clutch, the rods give way and punch through the crankcase, I think the clutch normally survives the experience though.
Just bad luck for you, I reckon!