Lithium, page-1375

Hi oz. The best info I have found does not blame the graphite. Seems most likely was a design flaw in trying to pack as much power as possible in the smallest space possible resulting in puncturing of the separator.

According to a unpublished preliminary report sent to Korea's Agency for Technology and Standards (obtained by Bloomberg), Samsung had a manufacturing error that "placed pressure on plates contained within battery cells," which "brought negative and positive poles into contact."
"The defect was revealed when several contributing factors happened simultaneously, which included sub-optimized assembly process that created variations of tension and exposed electrodes due to insufficient insulation tape," a Samsung representative tells CNET.

Or, in plain English: the thin plastic layer that separates the positive and negative sides of the battery got punctured, became the shortest route for electricity to zap across the battery (that's why they call it a "short-circuit"), and became a huge fire risk.
What does pressure have to do with it? MIT materials chemistry Professor Don Sadoway explains that today's cell phone batteries are made by literally pressing together a stack of battery components -- and that battery companies are under pressure (no pun intended) to cram in as much battery capacity as possible.

"Imagine if you had a toilet paper roll and it wasn't packed tightly," says Sadoway. With the same size roll, you'd run out a lot quicker.

The graphite theory I think comes from previous stories attributing metal particles in the electrolyte puncturing the thin separator.