Didn't the Bush regime use "imminent threat" as a reason for going to war with Iraq? With a major military exercise (including long range stealth bombers hitting close to her borders) can't North Korea see an "imminent threat"? After all it was military exercises that North Korea used as a cover for their invasion into the south half a century ago. Why can't they prepare to defend themselves from this imminent threat and use all sorts of sabre rattling as a warning (as a number of countries do in similar circumstances?
Of course, I may think they are just totally nuts but that doesn't mean they might not have very sane reasons for their actions. The US is overwhelmingly the world's most powerful military. They have demonstrated over and over that they will use that power if it is in the interests perceived by them. Those interests have never been clearly based on altruistic motives that I can see.
The various US administrations have made it very clear that they don't like North Korea so the US represents an clear and present danger to North Korea.