In 1947, The Bulletin of the Atomic Scientists first published the Doomsday Clock meme that defines their idea of the risk of nuclear war. The initial setting was 7 minutes to midnight.
The time got moved closer to or further away from midnight as the Soviet Union, China, and the United States built more weapons systems, had crises, or improved relations. The Doomsday Clock is arbitrary and there are no published metrics as to the setting. The Clock is now at 2 and 1/2 minutes to midnight, as close to midnight as it was in 1953 when the U.S. and the Soviet Union were in the midst of the Cold War.
It might be that the Board is considering North Korea, Pakistan, and Iran. The possibility of an Islamic nuke attack somewhere in the world is real enough. And there's no controlling the North Koreans, at least not by any means tried by the last several Presidents.
But my money is on the likelihood that the Board just wanted to express their dislike for President Trump.