The usual problem in these cases is internal corrosion, probably due to water build up. A manufacturing defect is unlikely since the bottle was hydo-tested before entering service, and on a routine basis thereafter. It should have had routine borescope inspections too.

Static could not ignite the contents inside a metal bottle...no flow path for the current. Also (except for really exotic applications like spacecraft monopropellants), flammable mixtures are never stored in bottles...otherwise ignition at the output would likely flash back into the bottle

Oxygen itself is not flammable or explosive anyway. It would be almost unbelievable if a breathing oxygen bottle were accidentently filled with a flammable mixture...there is a higher standard of handling for breathing and medical O2.

If the cyclinder was composite, and not all-metal, it's possible that the composite wrap was damaged by a sharp impact or by corrosive fluids.

If the cyclinder was weakened significantly it's possible that the cyclinder failed at a pressure below the relief valve setting.

Another remote possibility is brittle fracture. Steel can crack spontaneously at cold temperatures. We design steel products to have a low or extremely low risk of BF (depending on the application), but it cannot be totally eliminated.

From an engineering perspective, this is very odd. Given the application (high mx standards, no salt water) the odds are against this event occoring. My guess would physically damaged composite wrap, but those bottles in the pictures look like aluminum from a distance.