original_query: What causes the stagnation point on the topside of the airfoil to move?
query: What causes the stagnation point on the topside of the airfoil to move to the trailing edge?
answer: The progressively increasing vorticity in the bound vortex, caused by the increasing vorticity in the starting vortex, increases the flow speed over the topside of the airfoil.  This causes the stagnation point on the topside to move until it reaches the trailing edge.