Usually it's because of 2 things:
1) not enough fat in the recipe. May need as little as a tablespoon or so more.
2) not enough support in the board used - especially if a sheet cake. You may have to double or even triple the board.
If it is a weak board then the cake is cracking underneath the buttercream...not good....make sure that you have at least 1/2" thick support under the cake......