That's not a hoverboard a true hoverboard would not make contact with the ground.
Well, they got nicknamed Hoverboard on the Internet. Actually they've got multiple names, depending on which company imports them from China and sells them. They are called "IO Hawk", "Phunkee Duck", "Swegway" or just "2 wheel self balancing scooter". And they sell at a wide range of prices, from $200 to about $1500... although I hope that the companies with the big markups (i.e. IO) do take more care of their products so that they are made properly and don't go up in flames...
Anyway, I've seen multiple reports of such boards catching fire as well lately. But they don't only do this while charging, but also while being ridden. Something must be wrong either with the way the Li-Ion battery is treated in this vehicle, or the electronics overheat (or both).
I noticed that on Amazon they sell a spare 36V battery which is rated for a current draw of 10 amps maximum... but the motors are rated for an output of 2x250 W, so their combined maximum output would be 500 W, which makes them draw even more from the battery... but the battery is only rated at 360 W maximum output! Granted, this is only the peak output... if you're only rolling on the flat, you will maybe use 100 or 130 watts, but when climbing hills, you may exceed the maximum rated output of the battery for a longer time.