Based on the CBS report:
NTSB: Asiana pilots say they used automatic speed controls for landing that went tragically wrong in San Francisco - CBS News
... which states the position of the FD [PF=on; PM=off], "
Looking out of the window" may have been a contributory factor.
A scenario that I have witnessed several times, in high workload, "rushed" approaches, is that following a call for A/P disconnect & "Flight Directors Off", the PM will fail to de-select their FD, because they are cognitively overloaded (visually monitoring a (recovering?) profile, completing checklist & ATC calls).
This simple omission is rarely caught if not acted on at the time of A/P disconnect, and is alone sufficient to prevent A/Throttle wakeup.
This is how the FLCH trap works (in practise) and was well explained during training.
Just to clarify:
With both A/P & both FD off, A/T re-engages in SPD mode (with an FD left on this does't happen).
Despite the A/T switch being in the Armed position (glare-shield), the A/T may not wake-up if certain criteria are met.
Disconnecting the A/T (thrust levers) may result in A/T re-engaging.