A periodic event is occurring, which is that a car is catching up with the slower queue and abruptly slowing down. Each cycle of that event causes the back of the queue to move back by one car's length in the slow queue. The position of the back of the queue after each cycle is like the wave front of a shock wave. If the queue has moved forward by more than a car's length (plus buffer zone) during that cycle, the back of the queue will move forward (downstream). If the queue has moved forward by less than that length during each cycle, the back of the queue will move backwards.