One (partial) explanation I found:
If at any time I announce that a nation or kingdom is to be uprooted, torn down and destroyed, and if that nation I warned repents of its evil, then I will relent and not inflict on it the disaster I had planned. And if at another time I announce that a nation or kingdom is to be built up and planted, and if it does evil in my sight and does not obey me, then I will reconsider the good I had intended to do for it (Jeremiah 18:7-10).
Here, God explicitly stated that prophecies are announcements of coming disaster or good. But the recipients of that prophecy can influence the way the prophecy is fulfilled. Announcements of disaster might be reconsidered if the people repent. And announcements of blessing might be reconsidered if the people begin to act sinfully.
This may sound strange at first, but it makes perfect sense once we understand that prophets were covenant ambassadors. God’s covenant required loyalty from his people and provided consequences both for obedience and for disobedience.
Some people think the main purpose of biblical prophecy is to predict the future, and certainly that is an element of biblical prophecy. But traditionally, a lot of people have spoken of prophecy as being both forth-telling and foretelling. Foretelling would be, of course, the prediction of the future. But the forth-telling is very important when you read the prophets, because very often much of what they say is not predictive in nature. They’re confronting the people with their sin, they’re accusing them of having broken God’s law; they’re calling them back to repentance — so we could say it’s hortatory. And I happen to believe that the predictive element is secondary, that what God is really trying to do in classical biblical prophecy, like Amos, Isaiah, Hosea, those kinds of books, is he’s calling the people back to a proper relationship with himself. And very often the predictions are contingent; they’re conditional. God is showing them what their future looks like if they don’t repent. And actually, that’s the last thing God wants to do is judge them. So he’s warning them, if you don’t repent, here’s what’s going to happen. But if they do repent, God may very well not bring that judgment upon them. Or in the case of a prophecy of salvation, he’s showing them here’s what your future will look like if you continue to obey me or if you come back. So it can be negative or positive motivation. So I think it’s very important that we merge the foretelling and the forth-telling and understand that’s what biblical prophecy is really all about.
— Dr. Robert Chisholm, Jr.
Now, it’s important to point out that sometimes God actually did intend prophecies to give his people a glimpse of a certain future. At other times, he was so intent on bringing a prophecy to pass as stated that he miraculously ensured that his people would act in ways that brought about its unmodified fulfillment. At times like these, the prophets explicitly indicated God’s strong intentions.
One way that God indicated his strong intentionality was by adding assurances to his prophecies. These might be words stating his strong intentions, symbolic prophetic actions, or even miraculous signs. Whenever this type of assurance accompanied a prophecy, it indicated that it would be harder for human beings to change the prophecy’s outcome.
Sometimes, we see other prophecies confirmed by promises in places like Amos 4:2, where God swore by his holiness; Jeremiah 49:13, where he swore by himself; and Ezekiel 5:11, where God said that judgment would come as surely as he lives.
When God swore by himself, he effectively removed the possibility of human responses thwarting the outcome of the prophecy. God’s promises raised the certainty of the predicted outcome to the level of the covenant itself. As surely as God can’t lie, he won’t change what he’s sworn by himself.
The fact that God sometimes reinforced his prophecies by assurances and promises ought to comfort us, because our Christian faith is grounded in the eventual fulfillment of biblical prophecy. Most importantly, we believe there will come a day when Christ will return to earth to judge his enemies and reward his faithful followers. We have hope that one day God will restore his creation and wipe every tear from our eyes. These prophecies have been reinforced so frequently throughout Scripture that we know they can never be revoked or reduced. One day, all these predictions about Christ’s return will come true.