Oh, sure the North Koreans are going to attack CHINA??? It will NEVER happen.
They did this prior to their last major invasion scare of South Korea. IMO the troops are sent there as a pretext to prevent Chinese invasion in response to an attack on South Korea. The reasoning being that China will attack from the north and grab as much of Korea as possible before settling with the victor of the North - South conflict.
I think people have the wrong idea. These troops are more likely there to help catch those fleeing NK into China.