I probably wasn't clear enough, but I didn't mean to say the entire event was caused by exposure to air. The lithium ions inside the battery expand when they come in contact with sufficient oxygen. That expanding mixture is very flammable, and creates a lot of pressure which sprays out of a break in the membrane. Yes, it needs an ignition source, but a spark is not hard to come by when a battery is compromised that way.
As I said, the batteries were involved in hard crashes, so yes there was internal damage. That's what compromised the membrane. When the first one crashed, it probably shorted on impact and ignited. The second probably shorted when I jiggled the cable and ignited.