Surprise in my Microwave

While my youngest daughter was warming up some left over baked oatmeal, she opened the microwave to stir it and found it like this! Weird huh?

These bowls are old, 17 years old to be exact! We have used these in the microwave many, many times over the year, so I am not sure why it broke in half. We didn't hear a loud "pop" or 'bang' so go figure. Any ideas?
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