How do you determine when it's time to water? It looks like the result of excess water in the soil for extended periods; and, a high level of dissolved solids (salts) in the soil might be a contributing factor. When your soil is too wet, it limits your plant's ability to take up water, which results in a drought response. The first thing most plants do when threatened with drought is start shedding leaves to conserve water, even if the roots are drowning in a sea of plenty. Too, high salt levels in the soil work in combination with an excess of water to further limit water uptake. Enough salt in the soil can actually reverse the movement of water into cells and actually pull water out of cells (reverse osmosis) in exactly the same way that curing salt pulls moisture from meats like ham/ bacon/ jerky, .....

When you water, you should water thoroughly, so at least 15-20% of the volume of water required to fully wet the soul exits through the drain hole. Then, the plant should never sit in the effluent (water that has moved through the soil and out the drain). That practice provides a pathway back into the soil for all the salts that were flushed from the soil when you watered.

In passing, it should be mentioned that root congestion causes loss of older leaves, as does decreasing light levels, but given the rotted appearance of the leaf stems, I'm thinking your issue is related to poor root health. You might lift the plant from the pot and inspect the roots to see what condition they're in, and start using a 5/16" wooden dowel rod (any hardware store), sharpened in a pencil sharpener to test the soil's moisture content. Stick it deep into the pot. If it comes out wet or stained dark from the soil, withhold water until it comes out almost completely dry. There will still be sufficient available water at that point to satisfy your plant's need for moisture.

I think you'll find the info below to be helpful.

Growing Overview

Sick or Dying Plants?

Al