Reduce watering in winter. Use soft, tepid water. Mist leaves frequently. Take stem cuttings in spring. The cause is dryness at the roots, even a short period of drying out can cause serious leaf loss.
Cold air is another common cause of defoliation.
Other possibilities are too much sun or draughts. Surround the pot with moist peat and mist the leaves regularly. Foliage Begonias Begonias in most cases are grown for their floral display, although the situation is reversed with Foliage Begonias. The original Begonia rex came from India, but the species no longer exists and all the colourful ones you see on display are hybrids. The off centre heart-shaped leaves are easy to recognize but the Foliage Begonias include many other shapes, stars, ovals, spears, etc.