Why does spoiler extension during touchdown cause a pitch up moment?
Pitch-up upon spoiler deployment would occur if the spoilers are positioned aft of the center of lift of the wing when in its landing configuration. In this case, the spoiler kills off lift on the aft portion of the wing while the leading side of the wing (in front of the spoilers) is still generating lift. The lift generated by the portion of the wing in front of the spoilers is then not balanced by lift generated by that portion of the wing behind the spoilers, producing a net upwards pitching moment.