I thought this was a trade post... Not a BASHING post.
... Flexing of ANY kind from the INSIDE of a vehicle can cause an alteration is a window ( Windshield or Back Glass... )
Now: Don't get me wrong - I seriously doubt that it shatters due to the sound...
** BUT **
.. If the flexing caused a hairline crack - it could have hit the " sweet spot " and shattered...
Ex: A Diamond is the strongest NATURAL mineral ( object ) on each - But they have a sweet spot the if you hit it right dead on ( which you personally couldn't do - so no need in trying.. lol ) - it would SHATTER.
ANYWHO!!!! - BACK TO THE TRADING!