The Korean War never officially ended in 1953. An armistice was signed because the war became a stalemate so the front line became the agreed border between North and South and the DMZ is 2km either side of it. This is just a continuation of the hostilities that started 60 years ago
. The North claim these western border islands should be theirs and this isn't the first time they have attacked the South over them.
http://en.wikipedia.org/wiki/Northern_Limit_LineThis attack would also have something to do with the new kid on the block in North Korea trying to show he's as much a Western-hating psycho leader as his father was
.
Last person to leave North Korea turn the one remaining light off
As an aside this will hurt Korea's 2022 World Cup bid as its main selling point to FIFA was building more friendly ties between the two Koreas including playing some games in the North.