what is causing this?

8 weeks into flower and my leaves look like this

8 weeks into flower--the real question is what do the buds look like? i'd guess they weren't fed enough N in the beginning of flower. you're in the home stretch now, so no worries.

K def. Add langbeinite (sometimes called sul-po-mag). About 1g per gallon.

Or, maybe the worst case of nute burn I've seen. (Or, salt buildup?).