Why would a battery burst into flame?

In several crash tests, the battery on a Chevy Volt began to heat up or burst into flame. The battery problems happened a few days or weeks after the impact and the National Highway Traffic Safety Administration is investigating. (One potential factor: NHTSA testers didn't drain the batteries of juice after the crash, which is recommended. As a comparison, it's standard practice to drain a gas tank after a similar collision. So this could be an issue of new technology learning curve.) If you want to better understand why a battery that's been through a car accident could burst into flame, I'd recommend reading this piece by the Midwest Energy News. It's a nice summary of how lithium-ion batteries work, and how they respond to damage.