I was driving down a highway in northern California, with the bike on a rack behind my van. I heard a loud pop, hiss, and plume of white smoke. Pulled over right away, and the driver behind me says my van is on fire. I grabbed a fire extinguisher and ran outside to see the lower battery was spewing flames. It melted its way off the bike (luckily it was the lower battery...) and I was able to extinguish it and push it away before any other batteries caught fire (there was another bike on the rack too) or I lost my camper van.
I'm not sure why it happened. It was not a hot day, the bikes were covered with a tarp to keep them dry and the road was not rough.
Keep a fire extinguisher close at hand...you might just need it. Could have been so much worse for me. I can't imagine the battery going off indoors.