Quoted from : nestle.com
The most common cause of a crack in the surface of the cake is baking in an oven that is too hot. The surface of the cake sets very quickly, before the inside of the cake has had time to expand fully. Then as the inside of the cake heats up and starts to rise it has to break through the crust to have the space to expand. This tends to give a volcano look to the cake with the inside of cake spilling out of the crack.Actual oven temperatures often vary from the setting on the dial and it would seem that your oven might be running a little hot. Try setting it a little below the temperature stated in the recipe next time you bake a cake. Also take care to bake the cake in the center of the oven, not too close to the top where the temperature is highest.Another cause could be over-dosing on leavener e.g. baking powder or baking soda, so always measure these ingredients carefully.