The backbenches are the seats in the British parliament where ordinary Members of Parliament sit — the ones who are not ministers or shadow ministers. If someone “returns to the backbenches,” they have lost or left a top job and sit with the ordinary MPs again. News writers use it as a place-word: “anger on the backbenches” means those ordinary MPs in a party are unhappy. Though actually, the furniture is real wooden benches, but people almost always mean the group of MPs, not the wood.