Initially when the bus is at rest our body also follows the same state. But all of a sudden when the bus starts to move the lower part of our body tends to move with the motion of the bus but the upper part rejects this state of motion and continues to be in a state or rest. This results in a sudden jerk backwards when a bus moves. Also a person standing in a bus will be in state of motion, and when the brakes are applied the lower part of our body comes to the state of rest but our upper part is in state of motion. Hence, we tend to fall forward when the bus applies brakes.