What causes an air bubble to form between buttercream and the side of a cake?
Usually it's due to frosting a too cold cake or not allowing settling time. I always freeze my cakes so they are very cold when I fill and crumbcoat. I then let them settle for about 3 hours on the prep table so they can come to room temp and I can smooth out any filling that splooshed out. Air and gas get trapped under the final coating of BC or fondant while the cake is coming to room temperature and they push out. If you see a bubble forming you can usually just pop it and smooth it out....tho most of the time it will settle back once you pop it on it's own.
Ahhhh, the dreaded and mysterious air bubble! I had one this past Sunday on a little rush cake that I made for family. I didn't allow the cake any time to rest between thawing-actually stuck it in the microwave-, crumb coating, and icing. I think that was the problem.
I think it has to do with some gasses escaping from the cake-you could call them cake farts, I guess.