I agree with khorton that there is no obvious reason for getting an out of trim condition at autopilot disconnect. The trim input purpose is to trim the a/c for the actual conditions. If I disconnect the autopilot at Vref just before landing (AoA about 6 degrees) I do not get a lot of forces in my hand.
Then the question is how is the application of possibly max power (firewall) affect the rotation along the y-axis. I do not remeber this from the Q400 simulator, I do remember that pulling the PLs to disc after landing generates a pitch down movement. This depends on drag from the propellers through the propeller axis but there is also effect due to removing the propeller generated air-flow over the wings. This is no proof that it is the opposite when doing the opposite since for instance after landing the main gears are also generating a moment.
Anyone more recent on the Q400 has an idea of what rotational forces there are when applying max power from a trimmed idle position?