Reasons:
1) As was said, the base board is not strong enough. You may need to double if not triple the board under the cake. When you glue them together turn the 'groves' of the cardboard so they are X shaped.
2) You probably need a bit more fat in the icing recipe. Please post the recipe you are using. Most likely it is too dry. You don't want to thin it by adding liquid but make it creamy by adding more butter or Crisco.
Cake board is most likely the problem. The cake has to have movement in it to crack.
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