The only time I have had a cork pop out of a normally-sealed bottle was when I left it in my car during a hot Texas afternoon (I know - what was I thinking? But I was on my way to deliver it as a gift and got sidetracked on the way). You know those horror stories about kids left in cars on hot days? Well, it was that bad, but it was a sad and red-wine-messy situation inside my car. Note: only one bottled popped, the other two with it were fine (though somewhat worse for the high temperatures/cooking).
Never had the glass break, even when corking, so I have no idea what sort of pressure differential would be necessary to break the glass.
Still, my theory remains: pressure buildup inside from re-fermentation after back-sweetening + warm temperature (=warmer wine than when bottled = greater pressure inside) + sudden drop in barometric pressure outside due to thunderstorm => high pressure inside and low pressure outside, along with tightly sealed #9 corks => glass on top/side of bottle (which is thinner than the glass in the neck, I think) breaks under pressure from *air* inside the bottle, because the bottles were on their side! And perhaps the breaking glass of one bottle damaged or created enough disturbance to another bottle enough to break it too!
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