It seems the succession of power is no clearly known.
Most people assumed that Il's 3rd son, Un, would take over, but now it seems that his uncle, Chang Sung-teak, might take over since Un might not have enough experience to run the country.
Ether way, nether really has secure enough power like what Il had, so they might try a show of force to solidify a base (up to and maybe inducing an Attack/Invasion of the south).