The fields quivering, the skyline a grimace, At any second to bang and vanish with a flap;
The wind is making the fields quiver and forcing the skyline to grimace (pull a strained face)—almost like it's bending the horizon of the world itself.
The whole world seems like it might suddenly be blown away or just disappear, almost like a scroll suddenly rolling up, or a tent getting knocked over and blown away.
The wind flung a magpie away and a black- Back gull bent like an iron bar slowly. The house
These lines further highlight the violence of the wind. It's smacking birds around and bending them—ouch. In another simile, we learn that a black-back gull looks like an iron bar as it bends. It's like something strong getting forced into a shape by something stronger. The wind casually tosses both birds around.
Hughes might be suggesting that the forces of nature are indifferent to life—either animal or human. These forces don't care what kind of damage they're causing. They're impersonal.