Speculation:
Either No1 Thrust lever was "Stowed" instead of the speedbrake, leading to GA thrust on No. 1 Engine - only a couple of seconds at that thrust would be needed, or whilst stowing the speedbrake No.1 Thrust lever was caught on the long sleeve shirt/cufflinks of the pilot stowing the speedbrakes.
Some airlines have procedures whereby no "cleaning up the aircraft" is allowed until the aircraft is clear of the RWY, in contact with GND control and at taxi speed. The origin of these procedures was a similar incident.
Don't know if that policy applies in AF or not.