Jeezz you were very lucky. I've seen a Li-Ion in thermal runaway...well nearly in runaway...seconds from it.
Apparently the cell density or distance between electrodes is so close this is one of the issues with Li...this spacer is easily damaged causing thermal runaway.
But ambient temp causing this is something I wouldn't immediately think of...a quick google shows them to be dangerous 130C and above so looks like these were out-gassing way before this.
I suspect these were some type of Chinese dodgy counterfeit...which we all buy, because there cheap! A lesson learned for us all.
