Why do it? Raising noise with false alarms, desensitizes the intended signal of an alarm, ruining signal-to-noise, as people slack off about responding to alerts. It also serves as a probe to see what an actual outcome would look like. There's no profit (no money), and no incentive to whip emergency responders up into a confused state, for most non-state actors.
Even SWATTING is usually more targeted, with the prank being played on a specific person. Sometimes SWATTING serves to distract the target from something under their control. That doesn't seem to be present here.
North Korea's hacks usually come across as sort of impish in a lot of ways. They seem to like the attention of getting into the news. Messing with something reminiscent of air raid warnings seems to fit the personality of their general profile, given their ballistic missile ambitions. Other state actors in the news lately, probably wouldn't be as interested in domestic civil defense systems in the U.S.
They (whomsoever is responsible) might be motivated to do something like this (if it were a North Korean team) given some of the sabre rattling going around this season. It rings of something that would score points with Dear Leader.
But then again, yeah, maybe this is just the typical sort of "because it's there" hack, and some script kiddie found his way into another cookie jar.