As a kid, I liked making extensively large trails of dominoes standing up. Knock one down and the rest would be set in motion. I remember wondering why they tipped over and fell instead of just sliding across the ground when hit. The answer to that is friction. Because of its orientation, when standing up, pushing against it won't just slide it over, but also cause friction to act at the bottom. Because the forces act at a distance from the center of mass, a net torque acts on the object, tipping it over, and causing the same thing to happen to all the dominoes in front of it. If the domino were just flat on the floor, it would just slide since the ground would stop it from rotating around it's center of mass. It's common sense really. It isn't like you can hit fallen domino from the side and expect it to just flip back upright.