Why do the bananas fall off the flower when the leaves open up and how to I prevent this?
Without pics, its hard to say. However, these are some of the reasons fruits fall off before maturing.
Finger stalk rot: if cut surfaces of the clusters and hands are not carefully protected with fungicidal paste, finger stalk rot, which begins in fruit stalks, or comb rot, which begins in the cut area of the comb of a cluster or a hand, may easily occur. This disease is caused by the mold species Botryodiplodia theobromae and Gloeosporium musarum. The rot begins at the fruit stalks and spreads to the individual fruits, causing them to fall off. The skin turns brownish and later almost black, and becomes slippery and wrinkled. In moist air the skin is covered with a gray-green bloom. This infectious disease occurs both on the plantations and during storage.
Anthracnoses (fruit rot, black rot, tip rot): these diseases are caused by attack by Gloeosporium musarum, Botryodiplodia theobromae and Botrytis cinerea. Small, slightly sunken round brown spots form on the skin of the fruit, which in the later stages change into large, irregular, dark, almost black spots. Light, salmon-pink pustules then form in these larger spots, in which new spores develop. These spots may occur partly on the underside of the fruit, partly at the flower or stalk end of the fruit. The pulp exhibits watery rot and in later stages has a brown appearance