>> The reports all mention that the crack became visible suddenly and for no apparent reason, after the headsets had been connected to the external battery pack and stored overnight (some in Apple's Travel Case) with the soft front cover attached. It is glass. The device was being charged, which means some heat gradient between the charging battery and the outside world. That thin 3d-shaped glass cracks under thermal…
Actually, the thinner the glass is, the less likely it is to crack due to thermal gradients. It is thicker glass which experiences greater stress. The equation for thermal stress is:
thermal_stress = (coefficient of thermal expansion) ∙ Young’s modulus ∙ ∆T
which for glass, works out to about 0.63 MPa per 1 degC temperature difference between the centre and edge of the glass. Thinner glass has less temperature difference, so lower stress. Furthermore, surface roughness and type of imperfections lower the amount of stress needed to cause a failure, so Apple's obsessive polishing makes the glass withstand higher stress than usual.