just guessing here but.....
with flap extended, the outer wing is much closer (if not already stalled) to the the stall than the wing forward of the flap.
ANY yaw or (even minor) aileron input at the stall with flap will more deeply stall the retreating or 'aileron up' wing (depending on the input - yaw or roll), create lift on the opposite advancing or 'aileron down' wing, and dropping the retreating wing accordingly.