I would certainly buy the lack-of-oxygen theory. I do wonder why more of this is not handled by the autopilot; if not contradicted, it could descend to a safe altitude with enough pressure to revive the pilots, if not contradicted, it could broadcast a distress signal (good idea if you're automatically descending into who-knows-where), if not contradicted, it could unlock the cabin door.
Automatically unlocking the cabin door seems like a relatively low-risk option.
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