If pest infestations can be ruled out, the following are possible causes of leaf drop in birch fig (Ficus benjamina):
Ficus likes to be kept evenly moist at all times. Leaf drop is often a result of too much watering (waterlogging), but the soil should never dry out completely. Humidity should not be below 50-60%.
Possibly for some reason the roots of the plant are diseased (also possible due to waterlogging). Unpot the plant and look at the roots. Dead roots can be easily distinguished from healthy ones: They are brown and squishy-rotten. Remove diseased roots and pot the plant in fresh, coarse substrate; if possible, cut out some of the shoots with yellow leaves.
The plant needs a very bright location even in winter; in summer it must be protected from direct sun during midday hours. The temperature should not drop below 18°C even in winter.
