Speculation nr 1001
Hasn't this accident written "Bangalore" all over it? Remember the early A320 days where an Indian A320 crashed short of the Bangalore runway due to, apart from some other factors, a lack of understanding of the flight guidance system/ AT system?
Subsequently, in my former airline, the use of the "open descent" feature (Boeing: "FLCH") was forbidden below initial approach altitude.
Thesis Nr 1001:
Descending in FLCH declutches the AT i.e. AT mode annunciates: "hold" (Airbus: "clamp").
The FMA reads, from left to right:
HOLD (!!!)-LOC-FLCH SPD (yes, I'm making assumptions that the 777 flight guidance system isn't that different from a 737-800's system)
Normally , a capture of the G/S and/or a capture of a set altitude (has of course to be set LOWER than the a/c's actual altitude), changes the AT "HOLD" mode to "SPEED" mode i.e. engages(!!) the AT.
All it needs is the omission of the setting of a lower altitude and the arming of the G/S.
Seeing the glide slope indicator come upwards, while intercepting from above, makes one adjust the pitch upward, the FD will point downward (ordering speed thru pitch down adjustment): confusion!
This confusion and crucially, realising the speed drop too late and not immediately, manually adjusting the power sets the scene.