It is normal. I actually cut all the bottom leaves that touch the ground soon enough after transplanting, when new grows started and bottom leaves start to turn yellow. They do not get enough sun when new leaves open up, so plant stop feeding them. And as soon as you cut them plant starts grow more new leaves.

While this does tend to be a normal occurrence, it can also indicate a nitrogen deficiency when the oldest leaves yellow and fall off.

Most likely it is as galinas says, but what are your watering and feeding habits?

watering them regular, keeping soil moist, but not over watered. about every week and half adding liquid food to water (which has highest number in nitrogen) i think its 12-8-4, plants are thriving, but maybe it is the whole lack of sun thing for those bottom leaves.