I  made a SMBC fondant covered cake and after delivery to the client, a huge air bubble suddenly appeared on the side.

It was a hot day, but the cake was delivered by me in an air conditioned car, shielded from the sun.

I was also a guest at the party, so I tried to pop the bubble, but the buttercream had become too soft to fix the fondant hole.

Any ideas why this happened? Is SMBC a bad choice under fondant? BTW I had used marshmellow fondant. Would a ready made brand like Satin ice behaved better?

Just starting out my business, any help would be really appreciated! Have a lot to learn in cake decorating!

Thanks!

I'm sure it wasn't a problem with the icing under the fondant. When air bubbles happen, use a pin of some sort. push pin, safety pin, a needle even. I usually keep some in my cake decorating supplies. Go into the bubble at an angle carefully, poke a small hole, then you can press the air out and the hold is virtually invisible. I've had to do this quite a few times. Works like a charm. 

The air bubble was quite big and the fondant looked expanded. When I tried to press it back to smoothen it, the buttercream underneath was too soft and that side kinda lost the smooth shape.

Where can such a huge bubble suddenly appear from? The decorated cake had been out on my counter for 6 hours and nothing happened!

It could have been any number of things. Maybe just a small something was off with the SMBC, or the humidity (I've definitely noticed humidity before when baking). I can't narrow down to just one cause since I didn't see it happen. I wish I knew what to tell you!

Sounds to me like there was air trapped either between the layers of cake if you torted, or between the SMBC and the surface of the cake.