[ktallett]

It is due to a gas leak that caused an explosion.

[timr]

It definitely wasn't known at the time I wrote my comment, and as far as I know right now, they haven't announced the cause.

Few things would create an explosion of this size, so it's a logical inference, but you never know.

https://www3.nhk.or.jp/nhkworld/en/news/20260728_30/

Local (i.e. Japanese language) news is speculating an LP leak.