Too much raising agent or not enough baking time can cause your bread to rise in the oven but sink once it’s come out. Always check if the bread is cooked by inserting a skewer in the middle. If it comes out clean it’s cooked. Bananas have a nasty habit of making your bread seem cooked when in reality, it isn't. If you're not entirely sure that the metal skewer comes out clean, bake for an extra 5 minutes to make sure it's cooked all the way through.