This is a problem because a low center of gravity is more stable than a high one. Think how much easier it is to tip over a long two-by-four board balanced on one end than a block of wood the same width but only six inches high. In order to stay standing, the center of gravity has to be over the bottom penny. If it moves out far enough so that a vertical line going down from that point doesn’t hit the bottom penny, the whole stack falls. And every penny you add gives more wobble, making it more likely the center of gravity will stray outside that zone, even for a moment. When it does: crash.