This looks like they may have created a stress point when cutting out for the slide out. It doesn't take much of a miscut in a corner like that to allow a stress crack to form. Another possibility but less likely imo, that wall may not have all of the support it needs (bad weld, popped screw head, whatever) and if the frame is not solid, the motor could cause enough stress moving the slide all the way out if the button is pressed and held for a bit (which is what LCI says you should do).