IPod nano explodes on flames, unusable

It seems that it is probably iPod nano, but it ignited spontaneously and it started to flame during charging, started to scatter up to the spark by raising the smoke. It seems that he hurriedly pulled out the USB cable and it seems he managed to get inside, but what happened to such a terrible thing ......?

Below are pictures of other tragedies.
Exploding Ipod

Although I saw it brought to a local Apple dealer after the flames stayed, it seems that such a situation is unexpected, it is not guaranteed anything. However, it seems to have contacted Apple's head office for the time being, but there is nothing to contact from Apple even after 3 days have elapsed. So it seems that we have made a page like the one above and made it public.

Is it because the battery is the cause or is it another unknown problem?