If you don't keep your loaf in your oven long enough to finish baking, the center may be too wet and sink as it cools. Excess amounts of leavening agents can also cause the bread to rise too quickly and then collapse in the middle. Measure carefully. Also, if you often have issues with baked goods falling, this may mean your oven temperature is inaccurate. Use an oven thermometer to double-check. It's worth noting that with this recipe, your loaf may sink just a touch after baking. It is a wetter batter, but that's what gives it nice moisture, too. Once you've flipped your loaf onto the cooling rack, it will literally fall back into place.