There are several things occurring in this case.

The extension of the flapperons increases the effective angle of incidence (and effective AOA), which increases the lift. However, in this case the flapperons are extended well past the point of maximum lift, and drag is increasing much more rapidly than lift. This being a low wing AC, the center of drag is now pushed downward, well below the thrust line and center of mass, resulting in a down-pitching moment.

There is also a contribution from downwash that is probably approaching the lift from pressure differential -- thus pushing the center of pressure aft & making the AC nose heavy.

Had the flapperons been extended at a much shallower angle, the pitching moment would have been upward.

Overextension of flapperons is highly dangerous -- it can cause extreme adverse yaw and aileron reversal, in addition to nose-down pitching in a low-wing AC.