It's possible that the Chinese may be growing wary of NKs growing nuclear and missile technology. With the leadership of NK being irrational, the Chinese may be realizing that the crazy, wild-ass, dictator over there is just as likely to use the technology against China as he would anyone else.
Also, since China considers itself to be the only dominate power in the region, they may be irked that that this unstable nut has the power to bring the entire world to war single handedly on any pretense or whim.