Fred Kaplan, after noting that North Korea has engaged in a number of naval skirmishes with South Korea over the past decade, takes a crack at explaining why they upped the ante and torpedoed a South Korean vessel two months ago:
Some speculate that Kim Jong-il may have planned the March 2010 attack as a show of strength, both to the Seoul government and to his own military commanders. South Korean president Lee Myung-bak had already — for good reasons — abandoned his predecessors' "sunshine policy" of outreach toward the North. Kim is also believed to be caught up in succession concerns — he is thought to be ailing and wants his youngest son, Jong Un, to be installed as his successor (just as he succeeded his own father, Kim Il-Sung) — and he may have felt a need to toughen up his image after the humiliation of last November.
....Who knows how this latest gamble will play out. Some speculate that Kim made the move, hoping that it would frighten the South Korean people into voting out Seoul's current anti-détente government in next month's elections. However, some observers think that Kim has been spoiled by the excess indulgence of the previous two administrations — not realizing that the last few years of northern belligerence have strained the patience of many southerners.
Maybe. As Kaplan says, though, "You may notice the phrases believed to be, thought to be, and may have in the previous sentence." Nobody really has anything more than a guess at this point.