American plans to move its combat troops away from the DMZ, and outside the range of most North Korean artillery and rockets, is seen by North Korea as a hostile act. By moving south, the northerners see the Americans as more likely to launch air strikes against North Korea because the north won't be able to retaliate with artillery against US troops on the DMZ. The US is also pulling headquarters troops from Seoul, the capital of the south, which is also within range of North Korean artillery. From the American point of view, moving away from the DMZ makes military sense as the US 2nd Infantry Division is better trained and equipped than South Korean units, and thus more useful as a mobile reserve, to deal with any North Korean breakthroughs in the event of a war. South Korean troops are better trained and equipped than the northerners, and apparently better fed as well. Recent refugees from the north indicate that even soldiers have had their food supplies cut back in the last few years. 

It's long been pointed out that Seoul is within range of  North Korean artillery and rockets. That's true, but only just. Seoul is now a vast city of ten million people that sprawls north and south of the Han river. Parts of the northern suburbs of Seoul are within range of North Korean 130mm and 170mm long range guns. But the biggest killer would be long range rockets, mainly the 240mm ones. But only a few hundred of the "thousands" of North Korean rockets and artillery are these long range systems. The biggest fear is that the North Koreans would make the most of their limited ability to hit Seoul by using chemical weapons. This would means firing shells and rockets with nerve or mustard gas.