I've had the same thing happen when I grow. I dont think its a problem however. When the plant gets older it doesnt need the fan leaves for new growth so they die off. I either take the leaves off by hand or let them fall off naturally. I have read that if you take them off it helps so that the plant doesnt have to focuse energy on the leaf. I recommend taking off the ones that are the worst, let the other s fall off naturally. Another thing that can attribute is heat. When you get towards the end of the plants life when the fan leaves fall of naturally, they also become more succeptable to heat... If you've been flowering for a long time, its not a big deal...However if you just started flowering than it might be a secondary nute problem.