When a force is applied to the lower block in a stack of two blocks with no friction between them, the upper block falls because there is no unbalanced force acting on it to keep it in motion with the lower block. Newton's First Law explains that an object at rest will remain at rest unless acted upon by an external force. Since the upper block does not experience any force from the lower block, it remains stationary while the lower block moves. The discussion emphasizes understanding Newton's Laws to grasp the mechanics of the situation. Ultimately, the upper block's descent is a direct consequence of the lack of friction and the principles of motion outlined by Newton.