I'm surprised lithium ion battery fire hasn't been mentioned anywhere (at least on here that I see).
That would be consistent with an accelerated fire and given the event a few weeks back on the Alaska Airlines flight where an iPhone randomly burst into flames it's not an implausible scenario.
For all the effort focused on lithium ion batteries in the cargo hold the number of lithium ion batteries carried by passengers in their electronics is overlooked.
Quote from AS incident:
"All of the sudden there was like 8-inch flames coming out of my phone," Crail told KOMO-TV. "And I flipped it off onto the ground and it got under someone's seat, and the flames were just getting higher and a bunch of people stood up."
The Alaska Air spokesman told ABC News, "The fire was quickly extinguished by our flight attendants, who are trained for situations like these."
It's easy to see how that could get out of hand quickly if the flames jumped to something else before FAs could put it out.