Fatigue. Poor engineering.
Your right Mountainman poor engineering, I have another chain guard but it is not one of the new ones, so I will either leave it off (illegal here)or make my own. As long as I keep the chain clean the bike looks ok without it.
If Im going to stop the vibrations I will need to put the original cams back in, but I have seen a few stock s/m that are running very smooth (no modifications) that have done in their chain guards. 
