A colleague here was on the flight. Also reported the rapid drop in temperature, accompanied by sundry Hail Marys and not inconsiderable fear among SLF, panic in some instances. Then stewardess appeared and instructed passengers to use oxygen masks. Alas, masks had not deployed at that point, but did soon after, so that may answer question about whether deployment was automatic or not. Then a smell of smoke (more than likely from the O2 candles, though SLF wouldn't know that) and more distress among passengers.
Colleague's still calming down so let me caveat this with a 'caution' in case his recall is a bit off at this point.