The glass is designed to be flexed, and it also has a thin plastic coating on the inside of the panel to prevent shards of glass from pinging off.
I have seen a lot of different sorts of breaks, but they all tend to be either a sharp sudden motion (i.e. being dropped), or having something left on the top case when the display is closed.
The glass is held in by strong double sided adhesive, but it can be removed as a complete unit without any problems when you know how.