This is why we always caution the people coming on here wanting to throw LiPo and Li-Ion batteries in their project. If you don't know what you're doing (and sometimes, even if you do!), the failure mode can be spectacular.
dpa [the builder] is one of the most experienced people around the robotics field. Has built several truly "classic" robots. In regards cause of fire, my friend said this
Talking of hobby robots dpa posted on SRS about the disastrous fire, which consumed nBot, caused by a short in the LiPo battery. So he rebuilt it (5 months I think) and now uses NiMh.
Of course, it took them a while to figure out what actually failed with the Boeing 787 batteries.