I deal with lithium batteries all the time with model helicopters, planes, and cars. All that needs to happen is a small hole in the casing, or overcharging, or over discharging, and they go off. They burn like a high pressure flame thrower. Ticking bombs. I know of many people who have had their LiPo batteries ignite while being stored, with nothing going on, no charging, not being used, they just went off out of the blue (internal short), one friend had his garage burned down.
The batteries in the KERS systems are basically overcharged and over-discharged during every race.