Well, I just tested twenty o-rings from multiple batches. The Nitrile and EPDM ALL had tensile strength double that of the Viton o-rings.
Then I tested Atomic Viton o-rings (of a higher durometer) and they were similarly low breaking strength.
I tested Scubapro 2-010 port plug o-rings, and they broke much higher (?EPDM?).
Re: the failed hose o-ring, the company that supplied the hose does a HUGE volume, so I doubt old stock.
The failed o-ring felt supple, and didn't look dried or cracked under the microscope. But as I suggested above, from the surface markings, it didn't look like one of my replacements. I wish I could tell what material from looking at it. The hose is advertised as "Nitrox ready" which may mean nothing vis a vis the o-ring, and just refer to the inner tubing. But maybe it means a Viton o-ring, too, which is beginning to fit my prejudice about that material
No causes that I could see in the bore of the second stage tube or 90-degree fitting. The 90-deg adapter is a DGX fitting, which I've been happy with, though I have disassembled each one I receive and put LocTite on the threads of the inner nipple to make sure it never disassembles.
If tensile strength bears any microscopic relation to tear resistance, then I conclude that a pinch on these Viton o-rings may be more likely to create a flaw that propagates to failure, the thinking being that the pinch is a microscopic pull on adjacent o-ring material, just like a tensile strength pull.