Its definitely not because its done Grade, look how loose and fluffy the buds are, they should still be growing. Its browning for another reason.
What are you feeding the plants, what medium are you growing in, and is it happening to one plant or all of them?
How often are you feeding them?
And did you flower these strait from clone? My clones usually go through a massive amount of veg growth in the first month, and they use a LOT of nitrogen, Iv had mothers start to grow a bunch of single point leafs, eventually they return to normal growth. I would imagine flowering from clone would only increase the major nitrogen deficiency from all the growth because of the low nitrogen in flowering nutes.