Like someone said it’s a natural thing.. the entire plants life it stores whatever npk and sugars it feels it’s going to need during flower.. once the plant later uses that stored up nutrients it discards the leaf.. that is actually a good sign if you have yellow fan leaves towards the end of flower means you used proper nutrients and didn’t t over feed or under feed. Plants looks healthy just be careful giving all these nutrients to fix a few bottom fan leaves and effect your buds taste. It’s 100% natural and normal!! Just my thoughts happy growing!