Exploding Bottles (welcome back from vacaction!)

[clight385]

It really depends on the strength of the glass, a bottle with a severe flaw might only take 30 psi to break, where a normal wine bottle should hold about 150 psi. Your spraying bottle might have been under more pressure. In your situation the lower bottle might have broken and caused the upper bottles to side off and break, the first break would be internal pressure and the others could possibly be from impact.
By examining the fracture origin (if you can find it) will tell you if the breakage was internal pressure or impact, they leave a distinct marking or finger print.

 

[winojoe]

Sounds like time to switch to champagne bottles & closures.

How full were the bottles? Maybe the wine expanded from the heat and had nowhere to go except outward. Usually the cork will pop, but not if the capsule prevented that from happening.