Sometimes losing its leaves on the bottom is a natural occurrence as it cycles through its life, and then other leaves sprout and take over the photosynthesis. If it’s not getting enough bright, indirect light, it may drop off, too. Other times, a sudden drop of a majority of the leaves can mean that the plant has undergone an environmental change, perhaps due to repotting or a being placed in a new position in your home. Some plants don’t like to be moved and protest by dropping their leaves. Wait several weeks, and during the growing season, water as needed and keep it on a regular fertilization routine. New leaves will sprout.