The Chinese are not happy with DPRK right now. A few months ago, DPRK allowed Russian companies to develop its rare earth metal deposits. This was a double whammy for China. China is having a near monopoly in rare earth metal production right now, and any increase in production from DPRK might dilute the Chinese bargaining power. Also, the Chinese thought that Kim would prefer Chinese mining firms to the Russian ones.
This new leader is not too smart. That is why millions will die.