Last November South Korea fired on and heavily damaged a North Korean ship that it thought wandered into its waters (something that the North Koreans have been caught red handed at, and apologized for, in the past).
Analysts are begining to conclude that the attack was a "tit for tat" response to that attack.
http://blog.newsweek.com/blogs/declassified/archive/2010/05/21/why-did-north-korea-sink-the-south-korean-ship.aspxAccording to a BBC report, South Korea's ship fired on the North Korean ship after the Northern ship allegedly trespassed across a sea border that the two countries dispute. The North Korean ship returned fire, but not before it had been set ablaze after being hit by South Korean ordnance. North Korea maintained that its ship did not cross the border, and demanded that South Korea apologize, the BBC said. The New York Times reported that a North Korean naval officer had died and that three sailors were injured in the incident.
The U.S. officials say that U.S. agencies are nowhere near certain that the deadly March 26 naval clash was indeed a North Korean attempt to retaliate for the November incident. But they said that this is as plausible an explanation for the March incident as anything. Officials declined to speculate as to whether such a retaliatory act would have had to have been authorized at the highest level of the North Korean government—presumably by Kim Jong-il himself, assuming he is healthy enough to still be in charge—or whether it is conceivable that a military commander at a lower level could have taken it upon himself to pursue vengeance.
While the evidence that the South Korean ship was deliberately sunk by a North Korean torpedo might in some circumstances become an almost irrefutable casus belli, most parties to the Korean conflict, including the U.S., Japan, South Korea, and, most important, China, appear to be going out of their way to proceed cautiously and not allow the situation to escalate into a potentially catastrophic larger war. How to respond to the incident—and to future potential provocations by the North's mercurial leadership—is expected to be high on the agenda when Secretary of State Hillary Clinton makes a major visit to China over the next few days.