How far into flowering did this happen? When plants are blooming/flowering, they pull needed nutrients from the larger fan leaves, which causes them to yellow and fall off. This usually happens at the end of flowering.
If you look at your new growth, it appears a lot darker green than older growth... have you changed anything over the last few weeks? The leaves shouldn't all turn yellow like that, although Sativa's tend to drop a lot of fan leaves prior to finishing. I'd lean towards a nutrient deficiency or nutrient lock, making something unavailable for the plants to absorb, causing the creeping yellowing...I looked closely and thought I saw some of the newer growth's tips getting lighter as well. A Ph of 6.0 is a little low for my preferences, but should be OK unless your meter is off. I wouldn't recommend anything under 6.0 Fox Farms products seem to be high on nutrients, so I don't think that it is a lack of nutrients, but maybe a case of nutrient lock, where the plant can't uptake the needed nutrition.