Cajón Mágico y Bubba

Looks like its just happening to the lower leaves. Which being on the better half of flowering is normal. Especially if you don't do any defoliation then the lower older leaves usually die off first. But I also noticed you have some tips that are burnt on upper leaves which is typically a sign of to much nutes. I would try cutting feeding back by half and see how she responds so you can pinpoint the issue.