I know it's an old post, but; there are many cultural considerations that cause leaf loss when they require the plant to tolerate conditions approaching or beyond what the plant is genetically programmed to deal with, which causes stress. Too wet, too dry, nutritional deficiencies or imbalances, insect herbivory, biotic/abiotic pathogens, temperatures too high/too low, soil compaction, root congestion, poor or extreme light levels .... can all act individually or in concert to cause falling leaves. In my experience, most common causes among growers of ficus who are fairly new to bonsai are over-watering and/or a recent notable reduction in photo load (light levels). Stress unchecked leads to strain, during which the stress is so severe it limits the plant's ability to produce more energy/food than it deeds for respiratory processes ..... to maintain it's living mass. This is an unsustainable condition for any organism and always leads to loss of viability unless corrected.

Your job, the grower's job, is to determine which cultural condition is causing the stress and correct it. When that occurs, another cultural condition will then take its place as most stressful. Ultimately, our job is a cycle of determining the current most stressful factor and correcting it as soon as possible and to the greatest degree practical.

Al