When you are off throttle decelerating the injectors shut off to save fuel, around 10 mph in 2nd gear the engine is spinning too slow to be smooth so the injectors turn on again to maintain idle and prevent a stall. In my car this feels as you describe, a small lurch as the engine starts generating power again to stay spinning often resulting in a bit of oscillation as the idle tries to stabilize. An automatic transmission will smooth all these out but the MT is a direct mechanical connection so the effect is amplified, my old pickup did the same thing.
Dead giveaway: watch the instant mpg meter, you'll see it reading real high, probably even max at 127mpg and around 20 mph it'll start dropping and will continue to drop until it stabilizes around 10 mph if you allow the car to coast there.
At least my two cents, best of luck.
