787 Battery
This is speculation, not rumor...overcharging the battery could be the problem, meaning the charger is not wellregulated.
One rumor is that a contribyting factor was altitude...hard to imagine that.
Li Ion batteries have been known to overheat, I don'y know if later technologies have eliminated that characteristic.
You have to rely on the engineering guys and the FMEA ( Failure Mode Effects and Analysis ) and testing.