Timing

When you look at the timing of the torpedoing of the South Korean ship, it was probably a decision of Un as a payback for a nasty ship-sinking incident the year before that South Korea deemed as an unfortunate accident. They opened a dam spillway and a NK touring ship was sunk down river. Many died. Oops. The DPRK is not a 'forgive & forget' kind of nation. Just because the Americans decided where the sea border is that separates the Koreas, it doesn't mean the the DPRK agreed to that unilateral decision. Both DPRK and the South are highly geared emotionally charged regimes with a stark and direct capacity to express aggression, grief and rage with succinct and usually proportional actions.