with regard to the theory that an oxygen bottle failed of it's own accord,well i'm fairly sure that they are pressure tested at manufacture to about 2-3x operating pressure so a bottle failing on it's own would be quite a rare event!
Having personally carried and fitted these to many BA 747/777 aircraft over the past 30 years,i have never known one to "let go" as it were!.
Personally i do not know the Qantas set up on the 400s,so it is possible that there are extra bottles fitted near to the damaged area,which i am led to believe has a galley unit directly above it.
Personally i would go for the following scenario.....Oxygen bottle fitted in position but has for some reason developed a small leak,(insecure fittings/seal failure etc),....contaminant from galley above finds it's way to bottle and "explosion" occurs!!