When approaching stall, why does the center of pressure move back?
The center of pressure is generally not the best way to characterize the aerodynamic forces on an aircraft since it varies with AoA.  That is why the neutral point or aerodynamic center is used instead.  

Having said that the center of pressure is the location of average pressure force acting on the wing or aircraft.  Mathematically it is found by taking the sum of integral of the magnitudes of pressure and shear stress acting on the surface and multiplying it by a distance (for an airfoil from the LE) on the upper and lower surfaces resulting in a moment at the LE, or reference point.  This is then divided by the sum of the integrals of the pressure and shear stresses acting on the upper and lower surfaces.  My explanation does not do it justice see http://web.mit.edu/16.unified/www/FALL/fluids/Lectures/f03.pdf

It can be somewhat complicated to calculate for an entire aircraft but in general for an airfoil it does move forward up until the point of stall or critical point after which it then moves aft.  See https://www.reddit.com/r/aviation/comments/2kibfo/why_does_the_centre_of_pressure_move_rearward/ for a good simple explanation of this.  

The movement of center of pressure is highly dependent on the configuration of the aircraft (airfoils are much simpler) and unless you are certain of the specifics of the configuration generalizing the center of pressure movement is inaccurate...