Changes in the environment and insufficient watering cause ficus trees to drop leaves. Fungus and lack of light can also contribute to a ficus tree dropping its leaves.
It is common for ficus trees to drop leaves when the seasons change or when the temperature and humidity inside a house change. To combat environmental changes, move the ficus tree so that it is not near any windows, drafts, air conditioners or heaters, and use a humidifier when the air gets dry. Water only when the top of the soil is very dry, and ensure the pot has proper drainage for excess water. Ensure that the tree is in a position to receive adequate light. Fungus shows itself with yellow or brown spots on the leaves, and a fungicide corrects this issue.