North Korea has been crossing sea border of South Korea three times already this year. Most recent annexation was last Saturday when North Korean patrol ship sailed 3.8 kilometers into Northern Limit Line (NLL). Even though it's nothing new and South Korean officials believe it was just a mistake, they are still worried that the numbers of these incidents are abnormal. Especially after they claimed the new president of South Korea, Lee Myung-bak, as traitor and imposter, these little incidents brought large attention to Korean officials.
It might be over reacting if South Korea or its allies retaliate for such violations; however, it is still important to note that how North Korea is continuously warning or, in other way, bothering South Korea to express their opinion about the new president. Furthermore, North Korea used this kind of strategy in 2006 when they tested long-range missile, Tepodong-2, to claim that U.S and its allies should take North Korea more seriously. In addition, indirect threats that North Korea often projects can put them in higher stance in terms of negotiation.