Deck angle and Captain's movements?
BARKINGMAD,
I assume that the captain made his way forward up the aisle, flustered and sleepy, while the aircraft was rounding the top of its flight profile - and thus he didn't have to climb upwards. On entering the cockpit, I assume that he sat down right away - and thus did not have to hold on as the attitude steepened into the stall.
If he'd woken earlier or later, or crouched between the flying crew, then maybe he'd have experienced some inertial cue from his body, and would have recognized what was going on. Perhaps also if he'd had to grab to hold on in the terminal-velocity nose-high descent rather than just sit in his seat?