I think this problem occurs when the cake is not coated properly with buttercream of whatever one uses, before placing the fondant on the cake. But as mentioned my Deah, the pin poke does help if this occurs. Hope this makes sense.
1) As cake cools and settles, it lets out a lot of gas. Letting a cake sit overnight or a couple of hours is best.
2) Make sure the surface you are laying the fondant over is evenly tacky or moist. If you use a crusting buttercream, I like to take a wet papertowel and gently brush it all around the cake being sure not to miss a spot - You can also lightly mist it with water. That way, the fondant sticks to every inch of the cake and no surface is left dry to trap air around it.
3) When you lay the fondant over the cake you need to smooth the top of the cake 1st starting in the middle then out toward the edges lifting the fondant as needed to let all the air out. Then start on one side of the cake beginning at the top then smooth and press the fondant against the cake smoothing it down to the base, then turn the cake slightly, start at the top right next to where you left off then smooth it down to the base and keep repeating until you end up right up where you 1st started - You want to work the air down and out of the bottom of the cake and from one side to the other - don't randomly press the fondant against the cake.
Hope that helps!
If you do get air bubbles, like Deah mentioned, poke it at an angle (never straight into it), gently press out the air and lightly rub the hole until almost blended away.
I try to think of putting fondant on cake like trying to put tint on a window. I have also brush crusted buttercream with a light coat of piping gel before fondant, worked like superglue, no slip, no bubbles