I wouldn't stress it man, if they are dying off fro the bottom up it is natural necrotic growth which occurs at different stages in the light cycle, in the last 2 weeks those plants have made a draaastic change in the mind set, before it was, get as big and bushy and leafy as she could...
Now the plant is under the impression that winter is coming and wants to make itself the most fuckable piece of plant in the world, which include maximizing flower and oil production to induce pollenation...with a lot of strains you will see yellowing or dying off of the lower leaves, this is a natural process which you can allow to happen, or increase the nutrients you are supplying your plant with to make up for this increase in development