decompression?
Hmm so you think they had decompression issues and tried to get down, which would answer my initial question: If they were unconscious would the plane not continue to fly straight for hours, under autopilot (like in Greece incident some years ago?)
But if they were conscious enough to start a descent, that might be an explanation.
However, if they were conscious enough to start the descent, then they were conscious enough to
contact ATC/declare Mayday/at the very least squawk 77000 which does not even require them to speak (if too weak to do so)
thy did none of that, so i am not convinced by the pressurisation idea.