There isn't enough information in the article to determine the cause of the fire. He does say he was sitting in bed browsing the Internet, so it's possible he had the laptop resting on some blankets, which blocked the vents and caused the laptop to overheat and shut down.
I haven't heard of MacBooks spontaneously exploding under normal conditions.
There're many temperature sensors inside a laptop. They are designed to shut down on high-temperature conditions, well before exploding. The blankets cannot be the cause of the problem, without positing a second failure of some sort to defeat the safeties?
> it's possible he had the laptop resting on some blankets
IIRC, Macbooks do not have vents on the bottom to be blocked in that way. The only vents on mine are on the hinge between the screen and the body, and the mechanism works so that the could really only be blocked from the top.
In my experience, resting a Macbook on something that's soft (like a blanket) so that the underside is blocked has rather catastrophic effects on its ventilation.
That said, it should not be possible to cause a runaway failure in the battery via the laptop overheating.
I haven't heard of MacBooks spontaneously exploding under normal conditions.