I've used the same buttercream recipe for years on my cakes, but recently I started experiencing it cracking badly. I couldn't move a cake without the crusted buttercream growing a spider-web of cracks. It wasn't terribly noticeable on white buttercream, but on colors it definitely showed and bothered me to no end.
I went on a search for reasons why and found a plethora of possible reasons and ideas for preventing the cracking. What finally worked for me? Adding a tablespoon of light corn syrup per batch of buttercream (or approximately 1 TBSP light corn syrup per 2 Lbs of powdered sugar).
Adding this little ingredient to my buttercream has helped stop the cracking issue. The reason? The corn syrup helps thin the buttercream and unlike water or milk doesn't evaporate as the buttercream dries/crusts - meaning the icing retains some elasticity that it wouldn't otherwise.