Malbim: When a person sees a Chidush, he investigates the reason. If he saw that something else preceded the Chidush, he will investigate if the prior matter was the cause of the Chidush. However, if a Navi will come to him and tell him 'know that this matter happening now will be the cause of Chidush Devar Ploni that will happen later', and it happens like he says, there is no doubt that it was the cause. You saw the evil that I brought on Yerushalayim - it was a great Chidush, many constant evils, and now it is destroyed [and you saw the cause].