9 But what follows? “My God, make them like a wheel”. This is fitly taken as meaning that they should be constant in nothing that they think; but I think it may also be rightly explained, make them like a wheel, because a wheel is lifted up on the part of what is behind, is thrown down on the part of what is in front; and so it happens to all the enemies of the people of God. For this is not a wish, but a prophecy. He adds: “as the stubble in the face of the wind.” By face he means presence; for what face has the wind, which has no bodily features, being only a motion, in that it is a kind of wave of air? But it is put for temptation, by which light and vain hearts are hurried away.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)