As tensions with North Korea have risen, the US has been eager to impose new sanctions on them. Since the US doesn’t trade with North Korea in the first place, that has mostly boiled down to sanctions against Chinese companies that do.
The ease with which the Trump Administration has made sanctioning North Korea mean sanctioning China is coupled with a lot of angry blaming of China from US officials, who still expect them to solve North Korea unilaterally. China is unsurprisingly viewing itself as a target.
President Trump campaigned on trade with China being unfair, and many see that driving their actions against China as part of North Korea policy. Trump has presented North Korea’s continued tests as an “embarrassment” to China, and suggests the US could cut all trade with every nation that trades with North Korea, which covers a lot of nations, but especially China.
There is little sense that officials expect this to accomplish anything. Rather, Trump came into the situation with an axe to grind against China, and found it a fertile field for doing that. China, however, is increasingly annoyed at being blamed for that effort.
Interestingly, President Trump has conceded at times that China doesn’t have the capability to force North Korea to unilaterally give in to US demands. Despite this, he and other officials repeatedly go back to that claim, and talk of pressuring China to get them to do so.