Broken Rear Engine/Gearbox Mounting

I was doing a three point turn and trying to reverse up a steep hill. I revved it to 2500 started to let the clutch in slowly and the rear engine/gearbox mounting arm snapped at the front where its bolted to the gearbox. Its a 2001 Yreg 1.4mpi with 130000 miles. Could the soft engine mountings have allowed the engine to rock back enough to snap it ? The dealer suggested a boulder might have been thrown up to break it which i can't believe happening. I'm just wondering if anyone has experienced this before.