One thing I learned about as a professional baker was something called 'oven spring". Normally applied to yeast risen breads and croissants, it still applies to a smaller degree to leavened goods. When your banana bread hits the oven at the higher temperature, the initial contact with the heat causes the baking powder to work and the batter to puff, giving your bread a higher rise. This happens in the first 5 minutes. Then you turn the temperature down and allow the banana bread to bake to perfection.