Older fan leaves turning yellow, falling off (during flowering)

in my experience(3 grows) it's normal. They have served their purpose of supporting the growth of that node(branch)until it was fuly formed no longer get the light they one did so the pant reasorbs the nutes from the leaf to make new leaves/buds in the full light.
 
oldhippie64 has got ya covered.
 

conradino23

Grow Journal of the Year: 2017 - Grow Journal of the Month: Sept 2017
Yeah, nitrogen being used off. Part of the natural process.