This might be a way of understanding it, though 'm not qualified to do so!! To slow down, there has to be a drag force. If we double our speed, drag increases four times, thus the opposite will occur on decceleration. So should adding a drag force (reverse thrust) at a higher speed (ie 140ish kts on landing) not have more effect than it would at a slower 40kts, but still be effective enough to get the aircraft to roll backwards if not removed?
Adding airbrake at 400 kts would show significant decceleration, add airbrake at 10 kts, you wouldn't see much.