Engine mounts may be also broken and let the engine+transmission twist/vibrate too much and brake the transmission mount. Also if you hear noice from drive shaft (after installing new mounts) it may be bent, out of balance, or just the bearings (or joints) that let it vibrate too much. But these should be very worn to destroy a mount.
Also with manual transmission (or hardened automatic) if you do hard launces they strain on the mounts, specially if the engine has sizeably more kick in it (compared to stock). When beefing up the engine (not just headers and " cold-air" ), it will just brake the next weakest link on the driveline (including the mounts), if they are not also up to handling the power.