Could have been the pilots purposefully nosediving to get down to an altitude where the humans could breathe oxygen before they all passed out.
From Wikipedia...
Pressurization becomes increasingly necessary at altitudes above 10,000 ft (3,048 m) above
sea level to protect crew and passengers from the risk of a number of physiological problems caused by the low outside air pressure above that altitude. For private aircraft operating in the US, crew members are required to use oxygen masks if the cabin altitude (a representation of the air pressure,
see below) stays above 12,500 ft (3,810 m) for more than 30 minutes, or if the cabin altitude reaches 14,000 ft (4,267 m) at any time. At altitudes above 15,000 ft (4,572 m), passengers are required to be provided oxygen masks as well.