Answer
Newton's First Law (inertia) applies here.
If the bus slows, you keep moving at the same velocity at which you were moving (faster than the bus's current velocity), and so appear to lurch forward relative to the bus.
If the bus picks up speed, you keep moving at the same velocity at which you were moving (which is slower than the bus's current velocity), and so appear to be pressed back in your seat, lurching backward relative to the bus.
Work Step by Step
This is discussed on page 26.
