So... of course... I had to look it up. I found it at banana.com.
Bananas possess a unique scientific phenomenon called "negative geotropism." As the little bananas start to develop, they grow downward--as gravity would dictate. Little by little, several "hands" or double rows develop vertically and form a partial spiral around the stem. As they take in more and more sunlight, their natural growth hormones bring about a most puzzling phenomenon, and they begin to turn and grow upward. - LinkThus this cause the curve.