That is the plant using up stored nitrogen in the largest leaves to help make your buds better as hydrocannabis says. If you were in week 3 of flowering (also as he said) it would be more of a worry but not at the stage you are at now. This is why it was smart of you not pull the large fan leaves as some people do.

I don't think you usually see it in hydro because the plant has just about every nute in the world stored to the max in them....I've had my hydro plants on nothing but RO water for 2 weeks at a time towards the end and they still stay as green as ever.