If you talk to South Koreans, many will tell you that North Korea routinely yowls, howls, and makes threats they will not and probably cannot carry out.
Some will also say, as TikerLikesRooster has said, that what Kim Jong-un is doing has gone well off the long-established choreography of North Korean blustering.
My view is that very similar things were said about Adolf Hitler in the 1920s and early 1930s. He wrote what he planned to do in Mein Kampf and then proceeded to carry it out. However, many in Germany and elsewhere in Europe considered him to be an inconsequential buffoon — until the SA and SS started killing political opponents and the tanks started rolling over the borders to other countries.
Kim Il-sung and Kim Jong-il were both “known commodities” with a long history of bluster and minimal followup in most cases.
That may prove to be true of Kim Jong-un, but that has not yet been established. What is clear is that North Korea has major problems and Kim Jong-un must do something to fix the mess. One of the few tools in his toolbox is the military and it looks like he may actually be willing to use it.
A further factor is that Kim Jong-un’s rule is likely not yet secure. As the world's only hereditary Stalinist state, North Korea acts in some ways more like the monarchies of the ancient Korean “Hermit Kingdom” than like a traditional Communist nation, and they've just had one “king” die to be replaced by a new “king” who has at least one potential competitor, a brother who was originally favored for the succession but currently is under the control of the Chinese in Monaco.
For a very long time, the Kim family has promoted a military-first policy, creating a different dynamic from a traditional Communist country in which the Army and the Communist Party are two competing poles of influence. I don't think anyone yet knows how much control Kim actually has over the North Korean military. Maybe Kim himself doesn't know. That makes Kim dangerous for the same reason that a caged dog or rat or even kitten can be dangerous; people in fear of their survival may act in unpredictable and even irrational ways.
Here's the bottom line: my guess, for whatever it's worth, is that Kim Jong-un is new, he needs to establish his authority, he sees weakness in the White House, and wants to act before the new South Korean president (who is the daughter of a former South Korean general and strongman, probably South Korea's toughest-ever president) has time to establish her own authority in Seoul.
If the Chinese decide they don't want to lead with Kim Jong-un anymore and want to install a Korean to head the North Korean government rather than letting North Korea collapse and be absorbed by South Korea, Kim Jong-nam is the logical candidate.
All of this is speculation, however. I don't think anyone really understands what is going on in North Korea, and I'm not convinced that even Kim Jong-un understands or is in control of what is happening.
Also, here's a graphic that may indicate why the North Korean military is not something to be taken lightly.
I give him six months perhaps before he is removed in a bloody internecine coup, the details of which will only leak out, and which will only be known in full when that hellhole on earth, the illegitimate DPRK, falls once and for all. There is no honor among thieves (let alone tyrants and murderers). This provocation to taunt the US, S. Korea and Japan (and the world community), at age 29, could well be Fat Turd’s first and last hurrah. He is defecating bricks out of sheer nervousness and is in way over his head no doubt, that is for sure, he has tightened his ring of personal security to levels I think never witnessed in the regimes of Kim Il Sung and Kim Jong Il. Tanks heavily fortified around all his personal installations, battle fatigue (automatic long guns, helmets) for personnel around him within a 3 meter reach, you name it. I suspect he is wearing a vest as well, but won’t save him if an expert can place a bead and then can get a double tap on his forehead. Poor, stupid kid. He should have known what he was getting into, and given it up to Older Brother to worry about. Raw thirst for power, and papa’s dying favoritism toward the youngest one, over the other two, the “loose cannon on the deck” eldest Jong-nam and the “gender issue challenged” second son, Jong-chol (disregard the one daughter, Kim Sol-song, part of the father’s security detachment during his “on the spot” visits wearing a KPA lieutenant’s uniform).