What is the ionic basis for after depolarization in a record of an AP?

After the initial rapid fall in the graph, the rate of repolarization decreases. This slowdown is called after-depolarization. Initially, the large concentration gradient allows a rapid efflux of K+ and the rate of repolarization is rapid. However, as K+ leaves the cell carrying positive charges, these positive charges get lined up just outside the membrane. Thus, an electrical gradient gets created across the membrane, in the opposite direction (outside to inside). This, coupled with the fact that concentration gradient narrows with K+ exit, causes net K+ exit to slow down. Hence, the rate of repolarization slows.