Troy:
Some of the following numbers are from my faulty memory and imagination (cork diameters are correct). Consider the process, don't get hung up on the numbers.
Corks start out somewhere between 21 and 24mm in diameter (21mm is a #7, 22mm is a #8, and 23/24mm is a #9). A corker takes that cork and squeezes or compresses it to 15mm for insertion. The cork then expands back out to say 19mm (the diameter of the bottle). If part of the cork is out of the bottle, that part will expand further (say 21mm) than the part in the bottle. Using a mallet to force a 21mm piece of cork into a 19mm opening may not work. Now under the right circumstances (good bottle, damp cork and/or damp opening), this will probably work. Under the wrong circumstances (weak bottle, everything dry), it may not.
Now as cbw said, if there is insufficiuent space for the air that is being compressed under the cork, then the air pressure may cause the bottle to break.
Also, if there is a weak spot in the bottle, especially a small crack, then the forcing may cause the bottle to break (crack to expand), when normal insertion may not have caused a problem.
I don't have a lot of experience with using a mallet to insert a cork. I do have a lot of experience with customer-supplied used bottles and a compressed air corker. I have seen more broken bottles than I ever want to. The above suggestions are based on some investigation done 3-4 years ago, and some mutterings to myself often while cleaning up wine and broken glass.
Steve