Couple of things may cause that, for 1 its in flower and using its stores of nitrogen in the leaf to feed itself. 2nd is the fact that you are using CFL, which has low foliage penetration factors. Those bottom leaves do not see as much light as the tops of the plants and are nearly useless as food factories(Hard to photosynthesize), so plant decides to suck em dry and use its top leaves to make the food. HPS lights have a nice penetrating light and still you will see some dead, dying,yellow leaves at the bottom. the rest of your plant looks great so I don't think its much to worry about.