I'm going to guess the sanctions were actually biting the people in the upper echelons of NK leadership. Probably due to the fact that the Chinese were actually helping out in enforcing the sanctions on their border with NK. I've read that North Korea is run kind of like a feudal society, so long as Kim Jong can keep the military leaders happy with luxury goods and keep the military relatively well cared for. Once that stops happening, it can probably cause some extra anxiety for Kim Jong Un.