I wonder if this is an Enviro airplane? Most of the fleet is by now, I assume?
I remember hearing stories (and maybe even encountering it myself, long ago) about Enviro systems not making full differential. I wonder if they were only making 2-3psi or something like that?
Just a guess-- I'm sure we'll never know, given the condition of the wreckage-- at least unless it turns out the airplane had a history of these issues or something like that.
The Sundstrand is pretty much idiot-proof, as long as you turn it on and it's working correctly-- and if it's not working correctly, even an idiot will realize it very quickly. I don't think Commanders generally have had many hypoxia accidents, unlike the 441, say.