> Given that the battery is external to the headset, any heat generated would presumably need to be caused by a background software process gone haywire.
Apple is very good at background software processes taking 100%-500% cpu (100% == one core in Apple land).
>> 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.
The battery is external, but mobile. The users may well have stored it inside the thing. Plus, it is understood that this device continues to draw power while not in use, which also suggests a possible thermal gradiant.
Apple is very good at background software processes taking 100%-500% cpu (100% == one core in Apple land).