It can be a lot of things - oven too hot, too much raising agent, unbalanced recipe... Although I find that most cakes crack anyway.
You can control it, however, by taking the cake out a bit before half-way though baking (don't leave the oven door open) and slashing the crust being formed with a knife. It makes it look more appealing.
To be honest though, banana bread is never going to win any beauty pageants. Either slice the dome off the top and flip it upside down, or slather it with icing of some sort.
Or do like I do: Slice a 1cm layer, soak it in flavoured syrup and use it as part of a more elaborate cake.
