It could be leaf spot. Examine the green leaves and see if the spots are there.
I have leaf spot on most of my plants, but one was resistant. I had a bit of a N deficiency in her, and the leaves that yellowed out DID develop some spots, even though the green leaves have no sign of infection.
I am guessing that she is resistant and the spots do not proliferate unless the leaf is being discarded by the plant. My other plants, though, show spots on green AND yellowing leaves.