The O2 Bottle
Consensus appears to be that the bottle in question is one of the newer composite bottles. This has an inner aluminum liner. The regulator would be expected to be brass.
During the 6 months the bottle had been in place I would expect it to have undergone many cycles through a wide temperature range.
The two materials at the threaded top of the bottle would be Aluminum and Brass. These have quite different coefficients of linear expansion (COE). Those being about 24 * 10^-6 per centigrade degree for aluminum, and 19 for Brass.
Cold Aluminum is very brittle and could become cracked over time in the circumstances.
The disparate COE of these metals at this critical joint may be a big factor.
I suspect that this experiment with lighter O2 bottles may be at an end very soon!