North Koreans (particularly North Korean Christians) have long sought to leave their repressive country for the relative freedom of South Korea or even China or Mongolia. One of the "underground railroads" which these people use runs through China, and the Chinese have been assiduous in tracking refugees who come in or through their country. They not only have arrested and repatriated the refugees, they also arrest those who help them.
Now, there is an international movement pressuring China to ease up on the refugees, who Chinese officials say come to China for financial reasons, not political ones. Read more.