Did you know........that a cake shrinks about a quarter of an inch within about 24 hours?? I didn't know that either, but according to Duff Goldman from Ace of Cakes, it does!
Hmmm...that would explain the buckling here (to the left, just above the yellow flower):
Truthfully, I have never noticed any shrinkage, besides this here cake. And it could have very well been the icing sliding off the cake from the heat. Who knows. Perhaps I've never noticed the shrinkage because the fondant hardens a little after a time and creates a sort of cast around the cake. The most likely case would be because I'm a pig and eat the cake as soon as humanly possible.
So what to do about it? Duff Goldman recommends when trimming the bottom of a fondant cake, trim it a bit higher than you normally would so the fondant will stay smooth and straight when the cake shrinks.