It might very well just be a whole lot of saber rattling. But, in situations like these things can escalate beyond anyones control very rapidly. The recent announcement by the north that they are going to launch a nucular first strike has got to have put a lot of military facility's on a state of very high alert.
Just my point WTF announces that they will do a nuclear first strike. Even Truman said that the Japanese would face "obliteration" if they didn't surrender. Even he who had a specific thing he wanted didn't tip his hand and tell them he had the bomb.
NK put every one on notice to warm up their missile interceptor systems. We have our laser spitting 747s patrolling the Sea of Japan, ABM bases in Alaska are now all spun up. The iron dome is ready. Best chance of them scoring a hit is to pretend to be doing a surprise missile test and then WHAM! Sorry Kim, you tipped your hand. All units are now ready to shoot down your missiles and the boomer boats are just waiting to level all your bases with trident MIRV warheads with a 15 minute response time.
Even b4 pearl harbor, the Japanese announced their declaration of war just a mere 2 hours before the attack. Bad atmospheric conditions prevented an alert message getting through to pearl. The knew they'd still catch the Navy with its pants down.
No no, this is just a attempt to scare the US public into not cutting defense spending. Kim will get his foreign aid payoff via china soon.