I had exactly this situation recently. PM's F/D remained on, with subsequent speed decay in THR IDLE. Notably it was also due to distraction from the radio. All it takes is one missed call. The error was trapped, however, by the PM, performing his primary task of monitoring, and thrust was increased as PF corrected under Airbus golden rules.
My point is that it only takes one small break down in communication or procedure for this situation to arise. For it to result in protection activation and approach to stall, however, requires a lack of FMA awareness, sufficient monitoring from the NHP, and aircraft control from the PF.
For reference, FO with 1600hrs Airbus.