looks to me that everything is going to fine for you.If there is a fluctuation or discrepancy within the plant it will take the energy it needs from the lower less useful leaves to stabilize itself.Overall, they look fairly healthy, I wouldnt read to much into it right yet,sure its enough to raise a concern, but I wouldnt panic.
I think if had something to do with over-watering it would show signs of stress everywhere.Excess nutes,whatever kinda nutes(NPK) it would show more on all the leaves.....could possibly be low N, if you have been providing the correct amount of N for them, then it is most likely be a Ph problem, because they wont be able to feed if that is wrong......but I would hold off just a little bit in trying to correct something thats already genetically encoded to fix on its own..