Bloggs, to my simple airplane designing mind, it's pretty simple.
Asiana was high and hot. They then input a FLCH command that would have made the aircraft climb (no idea why they would have done that, but according to published reports that's exactly what they did) so the throttles started to advance. Since advancing throttles would be contrary to capturing glideslope when high and hot, the throttles were manually retarded and held at idle for several seconds which, by design, caused the A/T to disconnect (EICAS message and aural "beep beep beep" alert). Little more than a minute later they were caught unaware when that disconnected A/T, which they had programed for a climb, failed to maintain VREF airspeed for landing.
If you design an autothrottle such that it doesn't allow the pilot to override it if deemed necessary, why the
do you even need the pilot?