Once one or more of the mounts fail, the engine is allowed more freedom to move about, placing undue stress on the transmission case at its mounting point. The best case is the transmission mount also ...
I've got a 1974 GMC Sprint project car. It was originally a 350 small-block Chevy with a TH350 trans, but I sold the engine and trans and installed a 454 [big-block] and TH400 trans in it.