Courtesy of google translate it is not clear whether the bus was the cause of the fire or was just inside a building that caught fire.
They do say "However, several explosions were heard. It was feared that the hydrogen tanks would explode, but this did not happen due to the safety system in the bus itself."
This suggests to me that it wasn't an issue with the hydrogen system on the bus it was simply in the wrong place at the wrong time - but thankfully the hydrogen didn't ignite.
