I can't see any pattern. If it was a single bad cell going off, I would expect to see a "damage gradient", no matter how the heat was generated.
I would also guess this was a single event. Had it happened numerous times I would think all the cells would have bulged. Each time it would have happened, heat would be generated, and then the whole pack would get heat soaked, and slowly cool down over time. Single event, stopped by the fire crew, and cooled down.
They have lots of smart folks working on this. I am sure they are going down a hundred different paths at the same time.