On Perazim (2 Samuel 5:17-21) and at Gibeon (Joshua 10:10) God did what the people hoped and prayed for: fought against the enemies of Judah. But now God will do something unexpected--something "strange" and "alien"--namely, fight against Jerusalem (29:1-3).
In commenting on this verse (28:21), Martin Luther made a sharp distinction between God's "proper" work of grace and God's "alien" work of judgment. But because God is wise and good, God's judgment, too, is for our benefit: "But when our flesh is so evil that it cannot be saved by God's proper work, it is necessary for it to be saved by His alien work"--that is, God must destroy our ungodliness in order that we might be saved (Luther's Works, vol. 16, pp. 233-234).
In 28:23-29, Isaiah uses a parable from the wisdom tradition to note how carefully God works, even in judgment. God does not destroy wantonly or for its own sake, but like a farmer, only so that the crop might flourish. The farmer does not plow (tear up the earth) continually, but only in order to plant. Like the farmer, God does not thresh to pulverize the grain, but only to free the seeds and grain so that it might provide nourishment. Thus, again, God's "alien" work of judgment (metaphorically, plowing and threshing) is done in the service of God's "proper" work of saving (metaphorically, planting and harvesting).
21 For the Lord will rise up as on Mount Perazim,
he will rage as in the valley of Gibeon
to do his deedstrange is his deed!
and to work his workalien is his work!
23 Listen, and hear my voice;
Pay attention, and hear my speech.
24 Do those who plough for sowing plough continually?
Do they continually open and harrow their ground?
25 When they have levelled its surface,
do they not scatter dill, sow cummin,
and plant wheat in rows
and barley in its proper place,
and spelt as the border?
26 For they are well instructed;
their God teaches them.
27 Dill is not threshed with a threshing-sledge,
nor is a cartwheel rolled over cummin;
but dill is beaten out with a stick,
and cummin with a rod.
28 Grain is crushed for bread,
but one does not thresh it for ever;
one drives the cartwheel and horses over it,
but does not pulverize it.
29 This also comes from the Lord of hosts;
he is wonderful in counsel,
and excellent in wisdom.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org