What Would Cause Banana Bread to Sink in the Middle?

The reason that many quick breads, such as banana bread, sink in the middle after taking them out of the oven is due to several potential factors, such as being undercooked, not having enough leavening, allowing the batter to rest too long before baking, or having too high of a ratio of wet ingredients to dry. Banana bread is a dense quick bread, so a small amount of sinking can be expected, but not so much that a divot develops in the middle.