I've made two wedding cakes in the last two weeks. I used doctoed cake mixes and earlene's cream cheese frosting. I kept the cakes refrigerated until delivery and set-up. Luckily, I got to hang around the reception sites for about an hour before people started to show up. At about an hour after set-up each cake developed a HUGE air bubble. Luckily, I just popped it and tried to smooth as best as I could, turned the offending tier so it wasn't noticable and TRIED to forget about it. BUT, what caused it?? Do cakes let off "gas" as they come to room temperature? Inquring minds want to know??

yes cakes do let off gas as the warm and also as they settle. If your icing isn't as air tight all around you will have less bubbles forming.

If you used cornflower instead of icing sugar to roll fondant out with then this causes gas to form and ultimately destroys the cake so only roll out with icing sugar when covering cakes. Wasn't sure if your cake was buttercream or fondant covered - hope this helps.