I was working on 2 cakes, one a 2 tier MMF covered, the other just a 6" buttercream. This morning I was about to finish up this 6". I was letting it come to room temp, when I noticed a bubble on the side. (the one on bottom thats shows inside of cake) Well I got curious, tapped it on bar couple times and more appeared. (the ones around middle of cake) The icing is literally falling off the cake. What makes this happen and how do I keep it from happening again? I've been doing cakes for several years, but never had this issue. Oh and the MMF cake is doing same thing when I take if out of refrigerator. I'm afraid to take cake anywhere, because I got a feeling it won't make it. Any suggestions would be appreciated. Thanks.

Wow! That's one big air bubble! I get air bubbles all the time too (not that bad), & I really don't know what causes them. I can see them forming usually after I've iced with bc. I just stick a skewer in and smooth back over it with Viva and a fondant smoother.
I'm sure you'll get several theories on what causes these, but I don't think anyone really knows. Some say it's from not letting your cakes settle after filling them, but I let mine settle over night. Some say it's from icing a cold cake, but my cake is not refrigerated before icing. I do use the big Wilton icer tip to apply the bc, so that may be the culprit for me. I'm not sure I'll give it up just yet b/c it makes icing so much faster.
Good luck on your search for the answer!

the only think can think of moisture under the fondant, did you put a light crump coat under the fondant, some people put some thing with some weight on the cake to settle the middle bc, an do are you pulling your fondant down enough, or do you think it might be a little to thick just some suggestions, or maybe do each layer of cake seperate it looks like a tall cake

How long are you letting your cakes settle at room temperature?

The bulge pattern on that cake looks very much like the cake didn't settle properly, and when you tapped it on the bar (countertop I assume?) you forced the trapped air out which basically blew the icing right off.

Your cakes won't properly settle in the refrigerator as that hardens the fat in the cake and holds in the extra air. By letting them settle for several hours or overnight at room temp (filled and, if you wish, crumbcoated), the settling can occur before it blasts a hole in your buttercream.

Now small random bubbles sometimes just happen. As lecrn said, there've been lots of theories thrown around and no one is really sure what causes the standard type of bubbling. In your case, though, it really looks like a settling issue.