If you're curious as to why your lug nuts might loosen, it seems like the two biggies would be:
1) A force applied opposite to the direction of tightening. When you want to take off a lug nut you turn it in the opposite direction (counter-clockwise) from the direction used to tighten it (clockwise). Is there a way that force could happen to a moving car? Maybe with supercar-level braking performance you might imagine a scenario where the lug nuts could loosen slightly, but even if that were possible it wouldn't affect both front tires (i.e., the front right lug nuts would be tightened).
2) Vibration - why is there a specified torque rating for lug nuts? If you want to find out, put your lug nuts on hand tight, drive it around for a few days, and see what happens. Note: if it's not obvious, please don't do that.
All the vibration associated with driving (road imperfections, slightly out of balance wheels, suspension slightly out of alignment, etc) can act to loosen up the lug nuts over time. Presumably the manufacturers have tested this sort of thing out over the past 120 years and that's how we ended up with some general specifications for wheel lug nut torque (e.g., most small cars are approx 85 ft-lbs) that should be tight enough to resist loosening from common vibration over the time span that the wheels are on the car (because you're regularly rotating your tires, right?).
So, did your car have some sort of weird vibration? You said they lubricated the front brakes, so maybe a front caliper issue was causing abnormal vibration. I would ask them why they think the front brakes were related to the lug nut issue.
The roads where I live are horrible. Often the best you can do is pick which small pothole you're willing to hit to avoid the really big ones. When I swap back and forth between my all season and winter tires and wheels I torque the lug nuts. Then a week later I recheck, and usually do it again a few weeks later. That's just common sense in my mind. Even a low maintenance car like a Leaf requires regular basic safety-based inspection and maintenance of items like the tires/wheels, windshield wipers/washer fluid, etc. That's part of the process of owning and operating a car. My wife drives the Leaf and I won't send her out in a car that I'm not 100% confident about.
If you're concerned about recurring lug nut issues it's not much effort to check them. If you don't want to do that and you're worried about it then sell it off. You'll never get a better price than right now.