An emergency descent following a decompression with structural damage must be done in speed mode, i.e., the pilot changes from Mach to speed in order not to accelerate the aircraft during descent.
The rate of descent is therefore much lesser than when done at MMO.
An emergency descent caused by a catastrophic event is a brutal event and exposes the pilots (and passengers) for a longer time to hipoxy especially if masks are not properly don.
We normally train the emergency descent to be done in two steps/loops: the first thing to do is to turn and pull the altitude selector to a lower altitude, then the heading, the speed knob and the speed-brakes.
This is done having in mind the need to start an immediate descent not loosing time with details (what altitude? What heading? What speed?)
Only on the next loop we fine-tune those parameters.
They may have failed to terminate the second loop, due to having lost consciousness.