why did my banana cake fall?

Where do you live - in other words, what is your elevation? You may need to adjust for altitude.

This may seem like an obvious question, but were your dry ingredients well blended before you added them to the wet ingredients? You could get a sink hole if there's a pocket of batter that doesn't have much leavening.

One reason it might have fallen is if the pan wasn't big enough for the amount of batter. Did it rise properly while baking? Another reason might be the oven temp. - was it accurate? If it was too high or too low, the rise/fall could be affected. Sometimes a cake can fall if jostled too much just after removing it from the oven