4 And as these things are said of the future, though the sound of the words is past, it follows: “Turn us, O God of our salvation”. That which he had just related as if it were done, how prays he that it may be done, except because he wished to show that he had spoken as if of the past in prophecy? But that it was not yet done which he had said was done he shows by this, that he prays that it may be done: “Turn us, O God of our salvation, and turn away Your anger from us.” Did you not say before: “You have appeased all Your anger, You have turned Yourself from Your wrathful indignation”? How then now do you say, “And turn away Your anger from us”? The Prophet answers: These things I speak of as done, because I see them about to be done: but because they are not yet done, I pray that they may come, which I have already seen.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)