If leaves aren't getting any direct light the plant may naturally discard them.
That's not to be confused with plants lacking in nitrogen though... I've been doing a little experimentation lately using more of a veg based feeding regiment through my entire grow and got great results. Then I decided to take it a step further, I replaced my bloom booster (General Hydroponics Kool Bloom 0-10-10) with high nitrogen veg nutes (Dyna Gro Foliage Pro 9-3-6). Long story short, I'm never using a bloom booster on cannabis again.