I have received wisdom. I have learned the answer to one of life's enduring mysteries. (Note: I have not TESTED this answer, but my source was pretty authoritative.) And so I'm going to pass this wisdom on, out into the blogosphere....
I now know why weeping figs, Ficus benjamina, drop their leaves.
Yes, I know this is astounding, but bear with me. If your fig has done this, it is not because you looked cross-eyed at it. It is not because you are a bad plant owner and it hates you. It is not because you spoke intemperately around it.
In fact, the only thing that probably happened was that you changed the amount of light it was receiving. Maybe you brought it indoors for the winter, or moved it across the room because it looked better there. Maybe you simply put up new curtains or took down the old ones. Or it may just be normal seasonal variations.
It turns out that weeping fig is especially quick to adapt to new light conditions. This is one of the traits that actually makes it a good houseplant. It adapts to the change in light by dropping about half of its leaves and putting out new ones that just happen to be completely suited to the new light situation it finds itself in.
As I learned, plant leaves can be adapted to either high light or low light situations. It has to do with how the intracellular structures with chlorophyll, where photosynthesis occurs, are arranged. Generally it can take a plant up to 6 months to change its leaves from high light to low light, or vice versa. Figs just do it fast, by dropping and regrowing them.
The key here is apparently to realize that the plant is somewhat stressed while it is changing out its leaves and to let it rest. Don't fertilize it. Be careful to wait until it is dry to water it, then saturate the soil completely and let it drain out. Don't let yourself respond like normal people do and say, "It's dropping its leaves. It must be dying! I have to save it by giving it extra food and water." You'll kill it with love, for sure, that way.
I'm so excited by this new insight that I'm ready to go out and buy a new Ficus or two. I left 2 beautiful big ones behind in Mobile, so I'm currently "without fig". It seems like a perfect gardening challenge for the oncoming winter months.