I've had air bubbles under fondant, and bubbles on the sides of buttercream cakes where the cake layers adjoin, but never one on the top of the cake.
The cakes were in the refrigerator until 3:00 and delivered to the party at 4:00 they were fine until around 7:00 when one of the developed the bubble. Sorry, I don't have a clearer picture, but you can see the cake in the back. What could have caused this? The customer did say the bubble went down on it's own about an hour after it appeared.
I have found that they happen if I don't get a good seal on the top of the cake before getting the sides to seal. I have always caught them when I am smoothing, you can sort of feel the looseness of the fondant. I would assume that as the cake releases moisture and it is trapped in the bubble, it will create pressure and expand. Perhaps the moisture and gas eventually get out and that is what made the bubble go down. But i'm just playing scientist!
These are iced in SMBC, not fondant. Reread my question and I wasn't clear.