Yep, like nytros said, N toxicity. Those lowers are going to die off as the plant gets older, don't sweat the small stuff. It doesn't look real bad, just give her ph'd water the next couple and you should be good if your ph doesn't shoot out of range. Keep an eye on that! When you start getting too much nutrients in the soil, salt levels build up and will wreak havoc on ph.
I will tell you something else, you got a ph issue on that NLxBB. She is showing Mag deficiency in some of those last few pics, see the yellowing around the leaf edges? Sure sign. Check your runoff bro and I will just about guarantee it is out of whack.