You must have had an apprentice plumber, not one who has been in the business more than a couple of years. I wish I had $10.00 for every one that has cracked spontaneously. The worst feeling is to be walking down the hallway, hear a "click", and have water pouring out of the bathroom. I have changed more toilets that selfdestructed, than those that were "damaged" by the user. Call them and tell them you want a "second opinion" by an experienced plumber, and they can call me if they want a expert opinion that it could, and probably did, happen.