Suggest reading the relevant chapters of Book #1 at FLY BETTER......
or page 2 of the smaller document: Spin notes at http://www.flightlab.net/Flightlab.net/Download_Course_Notes.html
A common explanation is simply that the wing centre of pressure moves aft at the stall - seems to be based on the excellent book Aerodynamics for Naval Aviators - see that bit about "... as stall occurs, the drop in suction near the leading edge cause the c.p. to move aft." but that's not true of all aerofoils e.g. NACA 63-415. In any case, the data presented is from two-dimensional wind tunnel tests etc so doesn't really address the real situation of a three-dimensional wing, especially with flow separation originating from the inboard root.