NTSB had 3 Volts catch fire as many as 3 weeks in storage after some crash testing. The first one was weeks after a minor side impact test.
According to a friend in the industry, lithium-ion batteries, like the one used for the Volt, are chemically unstable. They tend to go into thermal over-run very easily. They are easier to manage in small packages, like the ones in your cell phone that contain only an ounce of material. But, when they are up-sized for bigger devices,like lap tops, they've had more problems. Sony can tell you that. http://www.msnbc.msn.com/id/15058478/ns/technology_and_science-tech_and_gadgets/t/sonys-battery-headaches-worsen/#.TtoTJHo2bSg
I can only imagine the danger and problems associated when they get super-sized, like in the Volt. There are plenty of videos on Youtube about exploding lithium-ion batteries. Even the small cell phone batteries release an amazing amount of energy when they burn. This is not a simple electrical fire.
PS. The Tesla seems to be avoiding these problems because of their battery design. It is basically thousands of small laptop Li-ion battery cells put together and protected with a series of sophisticated buffer circuits to provide a lot of protection. The Volt only uses a couple hundred battery cells, so they are considerably larger cells that are harder to manage.