I replaced many of those mounts. Usually the rubber remained bonded to both pieces of metal and tore through the rubber. There seemed to be a combination of causes, such as : rubber softened by oil leaks ( Corvair oil leaks weren't so much -- nothing has leaked more oil onto the American roads than the SBC oil pan and valve covers) , annual increases in engine power, and hard take-offs especially with MT cars.
The Jalopy Journal
