You need a pin to gentle pop the air out then smooth.
How soon are you putting the fondant on after buttercreaming up the cake? You should get the cake made up, then give it 24 hours to "sigh" (... when the cake is layered with filling etc it needs time to settle from the weight/moisture changes, If you fondant it before it has a chance to do this you will often get those ungainly "bulges" around the sides where the cake has settled after the fondant has been laid.) If the bubbles are on the top of the cake you want to ensure you have a smooth surface and then smooth the fondant outwards from centre (lightly to begin with) with a smoother tool, giving the bubbles a chance to escape down the sides x
The bubbles can come from 2 sources: air trapped inside the cake or air trapped directly beneath the fondant.
Air can be trapped INSIDE the cake while torting/filling/crumb-coating. Try to insure your filling has no gaps and literally press down upon each filled layer after placing cake layer on top. Allowing the cake to "settle" (as mentioned above) before covering with fondant will also help. If there IS air trapped inside; as the cake AND fondant warms (usually to a temp higher than room temp.) the trapped air will expand and try to "escape"; through the now-soft, warm filling & fondant; hence the 'bubble'.
Air can also become trapped directly beneath the fondant. Insure that you have a very smooth surface prior to covering, and insure that you maintain the fondant conforming securely to the form of the cake as you drape and smooth.
Sadly, it's happened to all of us! Sometimes, you can recover quickly with a pin; other times, more decorative work is needed to cover up the torn/expanded fondant. After pricking with a pin and smoothing, get the cake into a refrigerator immediately to cool/firm the fondant and prevent further tearing. It's always a head-ache and always best to do everything you can to avoid it! Hope this helps!