Revenge, most likely. The Koreans were treated like animals when the Japanese set up their empire in Korea and Manchuria before and during WWII. Korean women were taken as "comfort women" by the Japanese Military and shipped to wherever the Imperial Army went. The Japanese conducted human experiments on the people of Korea and Manchuria, including releasing diseases to see how they reacted in large populations. They made the Nazi's look like pikers.
How? Small boats and submarines. The North Koreans have small, but very good subs.
These historical issues are still big between the two Korea's and Japan, not to mention China and Japan.