For the benefit of non Airbus Pilots, the thrust levers are normally in the "CL" or climb detent throughout the approach. This allows the autothrust to command any power setting from idle to climb but the levers don't move whilst A/T is engaged. At the speed they were going on final, if the approach phase had been activated (automatically or manually) the engines would have been at idle, approach idle if flaps were deployed.
If the approach phase isn't activated, the A/T defaults to 250 kts below 10 000'. Many pilots have been caught out by going from selected speed into managed speed on approach without confirming that APP phase is active. A sudden power increase results as the system targets 250 kts regardless of flap/gear configuration. Doing it once is usually enough for most pilots and few repeat the mistake, as the master warning going off during the overspeed and subsequent visit to the Chief Pilot leave a lasting impression.
It's possible that they dived down from above the profile with A/T commanding idle thrust and if the APP phase wasn't activated, as the speed dropped below 250 kts there was a sudden increase in thrust.