Are you decorating immediately after you fill it or are you waiting? You need to let it settle before you decorate. There are a few different ways you can do it but it is imperative so it doesn't settle after covered.
The cake is going to settle. Period. Insist that the cake do it on YOUR terms. Settle before BC, after BC, and after fondant.
I don't freeze my cakes so my timeline might be different from yours, but I let my cakes cool for several hours. If it is cold outside, I will sometimes let it cool out there in a protected place, to speed things up. Fill the cake and stack the layers (not tiers.) I will often place the cake pan on top of the cake and put something heavy (canned good or sugar bowl, etc.) in the cake pan. The cake pan works the way the ceramic tile does, spreading the weight equally across the cake top. After half an hour or more, I will ice the cake. If I'm applying fondant, I'll do it then. After the fondant is on the cake, I let it sit for a while longer. If the cakes will be stacked, I will come back the next day and stack so the cake and fondant has had plenty of time to settle. Then I will decorate.
Followed one of the links that whoknew posted and came across an interesting comment about cakes settling..after the cake is iced and before it is stacked to run your spatula blade between the cake and the cake board and lift slightly to release any trapped air..
Found this interesting as it is something I have been doing just as a matter of course...lucky me..
Make sure your dam is made from really really stiff frosting and piped around about 1/2" in from the edges of the layers. After placing the top layer on, kind of press it down and then using that same frosting, pipe around where the layers meet to fill in the gaps. Then smooth it off with your spatula. I freeze my layers first, then fill and crumbcoat and keep in the frig to thaw gradually for several hours or overnite (loosely wrapped in plastic wrap) Then the final coat goes on and decorating done. Mind you, this is for buttercream frosted cakes. I don't work with fondant.