First time in 5 years that this has ever happened to me!  Made 6 baseball themed buttercream cakes for a senior recognition event.  6" double layer cakes on 8" cake rounds.  Crusting buttercream recipe = 2 cups shortening, 2 cups butter, 4 lbs powdered sugar, vanilla and 6-8 tbsp whole milk. Cooled, crumb coated and refrigerated one night.  Frosted (smoothed beautifully) and refrigerated the next night. Planned to decorate the following day prior to pickup, 2 of the cakes had cracked and sagging buttercream!  I always use the same crusting buttercream recipe with no problems.  Any ideas?

Not sure if you mean the cake cracked apart or the icing?                                      Did you have them on sturdy bases?  If it was the cake were they leveled?  What filling was used?   As you can see we need more info to be able to reply.     

The cakes did not crack, but 2 of them ended up with tiny cracks in the crusted buttercream on the sides and a few bulges on the bottom. I level all my cakes, they were filled with chocolate buttercream. This wasn't a filling bulge, but almost as if the icing was pulling away from the side of the cake. Same bases I always use with no problems.

Another issue I  had several years ago was using too much homemade cake release in my pans.  The oil in the mixture, which was on the outside edges of the cake, were causing the buttercream to come unstuck.  I looked like an air bubble, but was literally coming away from the sides of the cake.  No amount of smoothing would make it adhere.

I can say it was most likely caused by the weight of the cakes and the cake board not being thick/heavy enough. If you’re cake board can’t support the weight, you’re icing will definitely crack, and possibly slide down your cake.

Thanks for all your help! Lots to think about