The unique 777 implementation of maneuver demand and speed stability in the pitch control laws means that: An established flight path remains unchanged unless the pilot changes it through a control column input, or if the airspeed changes and the speed stability function take effect.
Trimming is required only for airspeed changes and not for airplane configuration changes.
Your particular situation is not covered in the available literature and I have not tried it in the simulator or the aircraft. Just a guess but would surmise that you have changed the reference speed so the auto thrust will increase to achieve the new ref speed.