It's almost springtime, that special time of year where the weather starts to turn, the flowers bloom, and the United States and South Korea hold the massive combined Foal Eagle and Key Resolve (formerly known as "Reception, Staging, Onward movement, and Integration" or RSOI). The Democratic People's Republic of Korea (DPRK) rattles its saber for two reasons. First, it will raise tensions whenever it needs something; money, food aid, or concessions from the United Nations, things of that nature. The second reason is the Foal Eagle/Key Resolve exercise.
Or to stop bad movies from being released. Where were you when Eagle Eye was coming to theaters, North Korea??
They have extensive external and internal intelligence and security agencies, as well as special units that infiltrate the South to establish pro-North Korea groups and political parties to foment unrest.