I made a cake this weekend that seems to have shrunk after it was frosted. Both layers were completely cool when I frosted them and looked just fine. The next day, the top of the cake seemed to have shrank lower than the frosting and I had to push the frosting back down onto the cake. Weird! Anyone ever have this happen or know what caused it? It was just a strawberry cake mix and the Wilton buttercream frosting.
How long had the cake settled before you frosted it? You can force the settling by squishing it.
sounds more like you got an air bubble (layer of air between cake and icing).....usually it will show up as a portion of the side of the cake or top, but I have had almost the entire top do that before. I use the wide icer tips to apply my icing and then smooth it with a spatula and sometimes i'll get a nice big air pocket in there.