An inflated balloon has a higher air pressure inside of the balloon than it does outside of the balloon. When a balloon is opened the air is released. The reason the air leaves the balloon is because the air pressures inside and outside the balloon are trying to reach equilibrium. This is where the pressures inside and outside of the balloon are the same. As air particles leave the balloon, they exert a force in the opposite direction. This causes the balloon to go in the direction of that force. The air that is flowing out of the balloon in one direction provides a force in the opposite direction. The direction of that force is the direction the balloon is going to move.