Icing cracks when the surface of the icing dries and the buttercream underneath is stays moist. Think of it like the earth's surface...you have the crust floating around on the surface of a liquid magma (earth here). Well, when you add too much water or milk to make your icing it does the same thing. Think of GLAZE...hard stiff dehydrated surface that is crusty and a thin icing under that...your buttercream is too much like a glaze...just thicker. When the cake shifts under it because of a bending board or the icing under the "floating crust" flows then it cracks. Decrease your liquid in your icing and increase the fat (butter or high ratio shortening) The fat will keep it soft but the dehydrating surface won't crust the same way or as much. The fat will make it icing without having to add so much liquid that you are making glaze. Also, using cream or butter instead of water will help keep the surface from dehydrating as fast and forming a hard crust. You want a crusting buttercream but not crusty one!