Breads and cakes begin to sink in the middle when the oven door is open and shut. This is because the oven temperature fluctuates and causes the bread to not rise and set properly. Your bread can also sink in the middle if it is undercooked. Be sure to test the center of your banana bread before removing it from the oven. If you see that the bread is browning too quickly around the edges, use a pie crust shield to cover the corners.