The article, unfortunately, doesn't really say why North Korea is threatening Guam. From the New York Times:
States have a hard time reading one another’s internal politics, so they tend to rely heavily on reading one another’s actions for clues as to their intentions. And American action toward North Korea remains unchanged. American troops in nearby Guam and Japan are still in their barracks. Naval warships are holding a respectful distance.
These are the sorts of signals, not a leader’s offhand comments, that matter most in international relations. Washington is sending a clear, consistent message to Pyongyang that the United States still wants to avoid escalation.
My personal hypothesis: by switching from a general, possible threat of war to a specific, actionable threat of specific actions, North Korea gets to sound out actual US posture with more precision than the off-handed comments of an inexperienced politician. As far as I can tell, we responded with leaked specific, actionable threats ourselves
The Pentagon has prepared a specific plan for a preemptive strike on North Korea's missile sites should President Trump order such an attack.
Two senior military officials — and two senior retired officers — told NBC News that key to the plan would be a B-1B heavy bomber attack originating from Andersen Air Force Base in Guam.
Pairs of B-1s have conducted 11 practice runs of a similar mission since the end of May, the last taking place on Monday. The training has accelerated since May, according to officials. In an actual mission, the non-nuclear bombers would be supported by satellites and drones and surrounded by fighter jets as well as aerial refueling and electronic warfare planes.
To be clear: "pairs of B-1s" "supported by satellites" and "surrounded by fighter jets as well as aerial refueling and electronic warfare planes" has been a mission we've been drilling since 1981 and carrying out since 1991. Maybe we've been doing more practice runs lately, but that's pretty much a symmetrical response to missile testing.
In short, North Korea is threatening Guam to see if we're all talk. So far, we're all talk.