I just read this article from Chemical & Engineering News 2007:
Burning Batteries | December 17, 2007 Issue - Vol. 85 Issue 51 | Chemical & Engineering News
probably nothing new to experts, but it confirms that it may be difficult to find a root cause for failures:
One fact that emerged is that it's tough to get solid statistics on the number of lithium-ion batteries that apparently explode or catch fire without having been set off by abusive actions. Unprovoked battery explosions are known as "field failures," and industry experts say such events are rare. They estimate that between one in 1 million and one in 10 million lithium-ion batteries fail that way.
Not only are the statistics of field failures difficult to pin down, but the fundamental mechanisms that trigger the hazardous events are also challenging to elucidate. For one thing, field failures are difficult to reproduce and study in a lab because they happen so infrequently.
Another difficulty in analyzing the causes of spontaneous failures is that batteries that fail in the field come from lots that have already passed abuse and reliability tests, and they appear to have worked normally for a while. Those batteries simply don't give researchers a clue that trouble is brewing inside of them. Furthermore, when one of them catches fire or explodes, not enough battery material may be left behind to determine what went wrong.