A few weeks ago I made a cake in buttercream. When I awoke the next morning it had a huge bubble on the side! What could cause that to happen?
Lastnight I made a cake and covered it in MMF over buttercream. The same thing happened!
It seems like the buttercream pulled away from the cake!
There are many, many posts about this problem on this site. There really is no simple answer expect that air/gas got trapped under the icing when you applied it. This happens all the time to many decorators. Some have found that by poking pin holes at the bottom of the cake (all around) it helps prevent/release it.
I personally found it happened mostly to my smaller cakes as one does not press the icing hard enough to make a good adhearance to the cake because the smaller cakes move when you try.
I have read that some people put tiles on top of their cakes once they cut their cakes and fill with filling for the cake to settle. I let my cake sit for maybe 30 to 40 minutes before frosting the entire cake. That way the cake settles and any air in between the layers will have come out of the middle by then. I am no expert but have read what others have done. Hopefully someone else will tell you another method for this issue. Good luck.