So hard to say. Looks like you're caring for it well, the soil looks good, it is off the ground, etc. Might just be naturally aging of the old interior foliage, or it might be some fungus. In a perfect world you would want to clean up the foliage to let sun and wind into the interior, which would make fungus less likely.
Never hurts to give it a good spray with neem oil, which is an organic pesticide with some anti-fungal properties. If you have had fungus problems in the past, I would consider getting a little more aggressive and treating with a rotation of Heritage, Clearys 3336 and Mancozeb (wait 7-10 days between each application and switch to the next anti-fungal in your rotation). This is a standard anti-fungal treatment we use on the west coast when cold and wet weather sets in.