Just guessing. . . Possible stroke/heart-attack given the age of the pilot, or as it has happened in the past, the pilot's O2 system malfunctions and he passes out (just guessing. . because that is what we do here). Regardless the cause, the pilot slumps over and his body pushes the yoke forward into a dive. The passengers in the back are NOT strapped in and float to the ceiling and never can get a hold of anything to pull themselves to the cockpit and take copntrol. . .or, they too, were passed out due to lack of O2 at altitude and never knew what hit them.
Just guessing (former fighter pilot and USAF mishap investigator, here).
My guess, cabin pressure issue, auto pilot off.
If the old captain vapor locked, the co pilot could take over. So doubt that.
28,000 ft is where things get real quick and dangerous in terms of altitude. Makes sense to me if the cabin didn’t pressurize, then that’s about where everybody would pass out. If auto pilot is on, plane keeps going till fuel runs out, like Payne Stewart. Auto pilot off, quickly out of control. BOOM!
(highly experienced pilot guess)