Here's an excellent overview of triggers of thermal runaway, mechanisms of internal shorting, and implications for managing field failures (particularly page 5 onwards, "Triggers for Li-Ion thermal runaway" from the annotated book facsimile) :
Lattice Energy LLC- Field Failures and LENRs in Lithium-based Batte...
I would be surprised if the 787 battery incidents were triggered by external "abuse", ie the batteries were operated outside limits, although it is not unthinkable. If this was the case, the investigation should be able to find out sooner rather than later.
It seems clear that cascading was an issue - due to inadequate spacing/thermal insulation, a failed cell would release heat to neighbouring cells. This will be adressed with the proposed new design (but as many have already pointed out, it won't prevent the battery from failing, it will just fail with less smoldering).
Boeing states that clean room technology is used to manufacture the cells. This is to minimize contaminations which could later trigger an internal short (such contaminations can not yet be detected with cell screening). I would speculate that cleanliness and manufacturing QC are well managed and not an issue (it shouldn't at least
, and if it is, the investigations should be able to find out).
According to Boeing, the 2.2 Million cell hours before January 2013 were without incidents. If the 787 incidents, or one of them, are true "field failures", as described in the book, neither triggered by external influence, nor triggered by contamination introduced during manufacturing, the investigations will never be able to know with certainty what exactly happened.
Now if Boeing wants to contain the fire, and hope it's a 1:1'000'000 thing, this leads me to believe they're leaning towards field failure as a probable cause?