I have been using fondant for a couple of years now and almost always have this problem. When I initially cover the cake, it looks awesome. Then, after sitting, it starts to form "bubbles" on the sides of the cake. I've read to use a pin to poke a small hole, which doesn't really work well.

I want to know what causes the bubbles in the first place! I use MMF on all my cakes.

Any help would be greatly appreciated.

There are some threads on the site about why it happens. Do you use filling or buttercream between the layers? Did you let them settle overnight before you covered them in fondant? After you fill and frost, the cake needs time to settle before you cover it. There are other arguments about whether you should freeze or chill the cake before covering. I'm not an expert so I'm probably not much help! Try searching for a thread about it because I had the same problem. HTH!

I believe these are like air bubbles causes by the natural gasses that occur inside a cake and when you cover them with fondant, the weight of the fondant forces them to "move" inside the cake and they have nowhere to go, causing the bubble.

It happened to me a couple times. I poked the hole and left it open for a while. Seemed to be fine after all the air works itself out.

I've found that it only happens to me when my frosting covered cake is really cold and I put roomtemp fondant on it. If I let my cake come to roomtemp and add the fondant then it doesn't happen.
But also, the needle poke trick works perfect for me. I poke the needle in at a really tight angle and sometimes even wiggle it around so it makes a little bigger hole than the needle itself does. Then because of the angled hole in the fondant it's really easy to smooth it back out. But if you poke it directly 90 degrees you will have a little hole that looks funny and is difficult to fix.