Looking back at photos of the Hudson plane, it's possible to see pins connecting the two plates of the HS pivot hangar above and below. So once the HS goes down beyond a certain angle, the bottom edges of its attaching lugs will collide with the lower pins, twisting the hangers backward at the lower end and pushing onto the APU separation ring.
Between the twisting of the hangers and the sudden downforce/drag, you suddenly have a whole lot of forces at work on the tail of the aircraft.