When the bottom penny shoots out, the upper pennies stay in a stack because any object that’s not moving tends to stay in place until an outside force pushes against it. This tendency is called inertia. When you flicked the dime into the stack, the force of the dime overcame the inertia of the bottom penny and pushed it out of place. But the inertia of the upper pennies kept them in place.
Photo by: Guy Cali Associates, Inc.