it could well be a combination of not getting enough light on the lower 1/3 of the plant. I had something similer wen i was bout 2 weeks into flowering. I made the mistake of pruning it all in one go, which set me back about a week or two cuz of the shock. I understand its ok to prune a litle ie. 10% or less. I think its best to do the pruning b4 u swich the lights, I could be wrong and if i am i hope someone corrects me but i think you should prune it back gradualy, gently so u dont stress the plant. It will divert the plants energy into the buds rather than trying to resurect dieing foliage
