What causes the glass to explode in a gas fireplace?

We had our Napoleon gas Fireplace running on Saturday night and after approx. 1-2 hours the glass front exploded and smashed into pieces over the rec room floor. Thank God, no one was hurt. The fireplace is approx. 10 years old. We asked a dealer about it and he said that is is a "B vent unit" which they are trying to ban in Ontario. Has anyone else heard of this?

http://advice.thisoldhouse.com/showthread.php?t=9685

http://answers.yahoo.com/question/index?qid=20071210115913AAb7qCP