The development of yellow leaves during flowering, especially at the very end of flowering, is typically a Nitrogen issue, starting from the bottom leaves and working it's way up a little. Even though the plants aren't putting on much height, they are still bulking up and need a lot of nitrogen to do it.
In my experience, it usually doesn't cause problems other than the yellowing, and adding nitrogen during flowering is frowned upon in some circles. I use 100%
promix with maybe 15% composted manure, so if you are using hydro, I can't speak from experience. Once they start to yellow, there isn't much you can do unless it's early in the flowering cycle. Sometimes the bottom leaves also fall if they don't get enough light...like when your buds are thick and light can't get into the bottom or interior of your plant. Sometimes the problem is a combo of the two.