Kids will almost always instinctively kick over (and thus back walkover) on the wrong leg if they aren't corrected. It is because it is easier to kick off the floor with the strong (good) leg, while your bad leg is in front, up in the air doing no work.
Yes, this is extremely common. I just work on both sides and don't teach kickover until they are strong enough and back walkover even later, and they will start to gravitate to their good leg.