Because of inertia. when we sit in a bus we'll be in motion with respect to us and our body doesnt want to change its state.this is due to inertia of motion. in the same way when we get down the bus, our body will be in motion and doesnt want to come to rest position immedietely. so we run for some time and slows down slowly and comes to rest position. this all happens due to inertia of motion. if we dont run after getting down, we'll fall down.