Howdy Bmw, I do not know what causes this, but I have had it early on in some of my grows. I think that it is due to some very small critter eating away internally in the leaves. I say a critter because, when I had this problem the first time, I could not find any answers for the cause and or cure. I treated my plants with a foliar spray of Azamax and also added it directly into the reservoir (I grow in a DWC setup) and after a few days, I began to see less damage on the older leaves and no damage to the new ones. Fortunately, it does not seem to be a 'catastrophic' infestation to the plant, but I do not want to let it go without treatment.
Good luck with Your grow.