Why then hast thou broken down its hedges? This is the application of the similitude; for nothing seems more inconsistent than that God should abandon the vine which he had planted with his own hand, to be rooted up by wild beasts. It is true that he often threatened and forewarned the people by his prophets that he would do this; but what constrained him to inflict upon them so strange and dreadful a species of punishment was, that he might render their ingratitude the more detestable. At the same time, it is not without reason that true believers are enjoined to take encouragement from such distinguished liberality on the part of God; that, even in the midst of this rooting up, they might at least hope that He, who never forsakes the work of his own hands, would graciously extend his care towards them, (Psalm 138:8.) The people were brought to desolation, on account of their own incurable obstinacy; but God did not fail to save a small number of shoots, by means of which he afterwards restored his vine. This form of supplicating pardon was, indeed, set forth for the use of the whole people, with the view of preventing a horrible destruction. But as very few sought to appease the wrath of God by truly humbling themselves before him, it was enough that these few were delivered from destruction, that from them a new vine might afterwards spring up and flourish. The indignity which was done to the Church is aggravated from the contrast contained in the words, when God, on the one hand, is exhibited to us as a vine-keeper, and when the destroyers of this vine, on the other, are represented to be not only all that pass by, but also the wild boars and other savage beasts. The word krsm, kiresem, which I have translated to waste, is taken by some for to fill the belly. [1] This sense would very well agree with the present passage; but it is not supported by the ordinary meaning of the word.

Footnotes

1 - "ykrsmnh, (jechar-semenna,) will destroy it Targum, Will tear it up with its tusk Fut pih From chrsm, he cut off, cut down, consumed, a quadriliteral, same as the Chaldaic qvsm. Occurs here only in Scripture, and, according to others, is compounded of krs, a belly, as though vkrs, will fill the belly from it." -- Bythner

Why hast thou then broken down her hedges? - Why hast thou dealt with thy people as one would with a vineyard who should break down all its enclosures, and leave it open to wild beasts? The word rendered hedges means wall or enclosure. Compare the notes at Isaiah 5:2.
So that all they which pass by the way - All travelers; or, wild beasts. So that there is nothing to prevent their coming up to the vine and plucking the grapes.
Do pluck her - Pluck, or pick off the grapes; or, if the phrase "all which pass by the way" denotes wild beasts, then the meaning is, that they eat off the leaves and branches of the vine.

Why hast thou broken down -
7. When a vineyard is planted, it is properly fenced to preserve it from being trodden down, or otherwise injured by beasts, and to protect the fruit from being taken by the unprincipled passenger. So God protected Jerusalem and his temple by his own almighty arm; and none of their enemies could molest them as long as they had that protection. As it was now spoiled, it was a proof that that protection had been withdrawn; therefore the psalmist addresses the Lord with, "Why hast thou broken down her hedges?" Had God continued his protection, Jerusalem would not have been destroyed.

Why hast thou then broken down her hedges,.... After having done all this for her; which signifies the Lord's removing his presence, power, and protection, from Israel; which were the hedge he set about them, and by which they were secured and defended from their enemies; but these being gone, they became an easy prey to them; see Job 1:10, the hedge about the church and people of God are the angels that encamp about them; salvation, which is as walls and bulwarks to them; and the Lord himself, who is a wall of fire around them; which may be said to be broken down when he withdraws his presence, and does not exert his power in the protection of them; but suffers them to be exposed to the persecutions of men:
so that all they which pass by the way do pluck her? the hedge being broken down, all passengers and travellers plucked the fruit of the vine as they passed along, there being noticing to keep them off from it: this may denote the plunder of the Israelites by their enemies, when left of God, they fell into their hands; and the havoc persecutors make of the church of Christ, and their spoiling them of their goods and substance, when they are permitted to do it.

hedges-- (Isaiah 5:5).

Hedges - Taken away thy protection.