From the article it seems to be analysis due to change in Pyongyang's behavior, coupled with intimate knowledge of the history of relations and personal experience with North Korea along with the current spate of aggressive attacks that seemed unthinkable by national actors. They're pointing to military preparations along with the fact that instead of waging an aggressive media campaign that usually accompanies their blackmail, they're being much more low key. Their language is much more vicious but less bombastic and attention seeking.

Some analysis of the troubling behavior from the NYTimes:

It has been evident for some time that something is afoot in North Korea. Kim invested his hopes in a 2019 summit with President Donald Trump in Hanoi — and that fell apart, leaving Kim humiliated. For decades under three leaders, North Korea sought a deal with the United States involving trade, prestige and economic benefits, but now it seems to have given up on that. Instead, it has bolstered ties with Russia, improved its nuclear weapon capabilities and escalated its rhetoric.

This week North Korea announced that it would take a much harsher approach to South Korea, changing its constitution and its longstanding policy on reunification, and would not respect traditional boundary lines. Kim said his army was making preparations for “a great revolutionary event,” which Carlin said is a phrasing that previously has been used to describe war with South Korea. Kim said North Korea did not want war but suggested it may be coming: “The war will terribly destroy the entity called the Republic of Korea” — the official name for South Korea — “and put an end to its existence. And it will inflict an unimaginably crushing defeat upon the U.S.”

I reached out to other experts to gauge their views. Joel Wit, a longtime North Korea expert at the State Department, now at the Stimson Center, said he takes Carlin and Hecker “extremely seriously.” Wit said that a recent incident in which North Korea fired artillery shells near waters disputed with South Korea “sent chills down my spine” because it seemed a possible rehearsal for a major provocation.

The Biden administration has not focused on North Korea for understandable reasons: It is wrestling with many other urgent crises. It may be too late to engage the North diplomatically if it has decisively given up on the United States, Wit said, but he added that China is now so deeply alarmed about North Korea that Beijing might be of help.

Deborah Fikes, a member of the National Committee on North Korea, a coalition of people with deep experience with the country, said that many nonprofits that normally have working relations with North Korea have been unable even to get responses to their inquiries. She, too, is worried about the risk of conflict.