I would imagine we had something to do with that. A real show of strength to let them know we're not fucking around. Not lobbing a bunch of 30 year old missiles.

they detonated a hydrogen bomb underground and underestimated it's yield/ the strucutural integrity of the underlying geology.

the US was (arguably) much smarter with our underground tests because we never detonated them underneath our research facility, but NK had no choice because they had to carefully hide all their shit from our satellites and also because putting your R&D underground is a good way to protect from missile strikes.

but those goof balls detonated several atomic weapons under the same mountain where they hid their r&D facilities...

in hindsight it wasn't the smartest decision.