Did you pH adjust the water you've been giving them before and during flushing? What medium are they growing in?

My guess is this is because you stopped feeding them and flushed. I would only flush for a week in soil, 3-5 days in coco/similar and 1-2 in hydro. It really doesn't take that long for the plants to use up their available nutrients. You're starving your plants to the point where their leaves are dying I think. Next time try waiting until you are closer to harvest time before you stop feeding them and flush.

You want to start flushing when the stigma & pistil turn darker and curl back, and when your trichomes are all milky white (for maximum THC) or when they have started to turn amber (for more CBD & CBN). They will continue to get less transparent and eventually turn a brownish/amber coulour. The absolute latest time I would wait to harvest is when ~30% of the trichomes are amber in colour. So if you are flushing,time accordingly depending on your growing medium.