As far as I konw, the reason behind this is because unlike a standard auto, the CVT uses a computer controlled clutch-pack, rather than a fluid torque converter, to prevent stalls. This means to keep the car from stalling out, it will open the clutch at very low speeds when not accelerating, allowing it to roll. I notice the same thing with rolling backwards slightly on hills when coming out of auto-stop in my HCH-I.

Holidn the brake or e-brake like might be done on a manual car on a steep hill to prevent rolling should work.