Anything that can be done to flare up tensions between these two adversaries then serves the US’s goals of sowing chaos and division to keep the things from progressing smoothly. Khan was elected to, in effect, drain the Pakistani Swamp. His, like Trump’s, is a tall order.
And at this point it looks like he’s still willing to give it a go as opposed to Trump who is simply revealing himself to be a thin-skinned version of Barack Obama, albeit with a distinctly orange hue. But, still why right now?