Sounds like metal fatigue, from the repeated pressurizing and depressurizing of the cabin.
It's not unheard of.
In the 1950's, early pressurized planes came apart, until engineers figured out how much pressure could be contained, and how to design the plane to handle it.
http://en.wikipedia.org/wiki/BOAC_Flight_781
http://www.dmflightsim.co.uk/dh106_comet_history.htm
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