Welcome to the forum.
Motor mounts do fail. Some cars are worse than others. Aggressive driving can also accelerate their demise.
Out of curiosity, who was it that said all four of your mounts broke? Was it a dealer or private garage? The reason I ask is because it is very odd to hear that all four have failed.
2005 SL AWD, Platinum
-OEM HID conversion & HID fogs
-OEM iPod interface
-Upgraded (non-Bose) speakers - Pioneer TS-A1702C & TS-G1643R
-Muth signal mirrors
-Valley Industries hitch and wiring harness
-Aluminum fuel tank shields