I don't know why the cb does this-I just know I have to find a different way to get back down table-a little cut/low running english if available.
It's really quite simple: when you impart top or bottom spin to a cueball, immediately after you hit the ball, the cueball "slides" on the cloth and rotates on itself in the direction you chose (top or bottom spin). Then progressively, that spin gets converted into a forward or backward force because of the friction between the cloth and the ball. When the spin "dies" completely (that is, when the ball finally rolls and spins naturally), the resulting direction of the ball is a combination of the initial forward movement, and the spin-friction-induced movement.
In the case of a simple force-follow of force-draw on a dead straight shot, it's easy to understand: when the cueball hits the object ball, its forward motion is transferred totally to the OB and stops dead. Then the spin-induced force re-accelerates the ball forward or backward. If the object ball is close, the cueball essentially slides and hits it, and most of the spin takes effect after the hit. If the OB is far, most of the spin has already been converted to forward motion, and merged with the cueball's original forward motion. The harder you hit of course, the farther you can slide, and the farther you can "bring the spin".
Easy to understand so far. Difficulties start happening because of spin inertia: if you spin a cueball iin a certain plane, the spin will stay in that plane. So if you hit an object ball 1/2 ball for example with tons of follow, after the hit, the cueball has changed direction 60 degrees. But whatever remains of the spin spins the cueball in the original spin plane, so the ball reaccelerates
in the original direction of the shot. After the hit, the spin-induced movement merges gradually with the new direction of travel of the cueball as the spin disappears, that's why you see the cueball curve from 60 degrees off the original direction of travel right after the hit, to something between 60 and 0 degrees after the spin is gone.
Likewise, if you draw, after the hit, the ball still exits the collision 60 degrees from its original direction of travel, but the backspin grabs the cloth in the original direction of travel and gradually reaccelerate the cueball in that direction, so it curves back to you.
So what happens with the OP's shot? the shot is not quite straight in but close, and the OB is close to the rail. If you shoot hard with follow, the cueball slides most of its way to the OB and hits it. Then the cueball transfers most of its forward energy to the OB but not all. It then exits the collision at say, a 80 degrees angle, with slow speed (most of the original forward speed being gone into the OB). Then it gradually re-accelerates in the original direction of the shot. Trouble is, the rail is so close the cueball hits it before all the spin is converted to motion. So the ball rebounds backward, but remember, the spin still tries to move the ball in the original direction of travel. So in the end, when the spin is completely converted to motion, it's fought the backward motion enough to kill all motion altogether, and the ball stops dead near the rail.
Simple eh?