FYI

http://www.ecozen.com/batdict1.htm

"Gassing -- Gassing occurs when more charging current is being fed to the battery than it can use. The excess current produces Hydrogen and Oxygen gases. Some gassing is normal, but excessive gassing can indicate that the batteries are being overcharged. The gases released are explosive if a spark or flame occurs, so adequate ventilation must be provided. This is even more problematic for sealed batteries due to pressure build up. Batteries normally start gassing at about 80-90% of full charge. A common fallacy is that you should stop charging as soon as the battery starts gassing. If you quit charging at that point, you will never get a full charge. Most good quality chargers reduce the current when the battery reaches this point to prevent excess gassing."

Could be a combination of an old battery and an agressive charging system, leading to a hydrogen bubble, and when the car was started, an internal spark in the cell ingnited the hydrogen.