It could be a number of reasons. A few older leaves at the bottom of the plant which has less light is not usually a problem, However yellow leaves could be caused by other problems. It could be overwatering or too little water. There may be a lack of nitrogen To trst this you could just add some additional feed and watch to see if the plant greens. It could be pests or it could be a fungal or bacterial disease. You need to establish the cause because the latter two could spread and put your whole crop at risk.