They want:
1. To save face for Kim Jong Un, who has escalated himself into a military/diplomatic jam and now needs to claim some type of “amazing military victory”, ergo the Kanagawa attack and these sub shenanigans.
2. Money
3. Food
All this is huge cause for concern (mines, shell exchange) but aside from a possible impending torpedo attack my guess is it’s not part of a general, theater-wide attack from the Tree Bark Soup set.
They want another 10,000 tons of fertilizer so North Korea can fake a big nuke this time.
There is not much middle ground when it comes to war between the Koreas. It is either a little bit of shooting back and forth or absolute Kaboom! Something like 20,000 missiles are readied and aimed at Seoul.
They want another check.