Almost any historical occurrence can be explained in multiple ways. For example, consider the results of any recent election and the variety of explanations of what caused the election and what it means. Sometimes more than one factor is a work. For ancient history, the amount of information we have is less and the variety of theories that can explain the available information increases. In such a case, a flimsy theory that has no rivals shows a lack of imagination on the part of scholars.
During the reign of Josiah, he renovated the temple (2 Chronicles 34:8-13). During the course of the renovation, workers discovered a scroll of the law of the Lord written by Moses (2 Chronicles 34:14).
Scholars have often equated the scroll with the book of Deuteronomy. To them the philosophy of history in Deuteronomy is similar to that found in Kings, whose history extends to the early part of the Babylonian exile. They therefore conclude that Deuteronomy was written about the same time as Kings, which must be about the time of the exile. Therefore, the scroll that Josiah discovered must be that of Deuteronomy which was written (forged) at the same time.
On the other hand, the philosophy of history in Deuteronomy is also similar to that of the Hittites who disappeared at the time contemporary with the traditional date of Moses. If Deuteronomy was written early, then the rest of the case collapses.
One alternate theory to explain what the discovery of the scroll is to remember that it takes only one generation for a tradition to be lost. As Jeffery R. Holland has pointed out, the Church is only one generation away from apostasy. Manasseh and Amon (Josiah's grandfather and father) were not righteous. There is no reason to think that they paid any attention to the law of Moses or encouraged its study or application. They are an example of a generation that neglected the scriptures. Their example probably spread down the line. Josiah had no positive example and need not have had any knowledge of the written law or any knowledge that the law was written before the discovery of the scroll. Written texts were comparatively rare, and knowledge of them seems to have been comparatively restricted in ancient Judah. So the scroll could have contained the Torah, as has been traditionally assumed, and this is at least as plausible an explanation as the current scholarly fad on the subject.
There are other possibilities that might equally explain the evidence. There is no reason to posit that the scroll was only Deuteronomy or that Deuteronomy was forged at that time. We are limited only by the evidence and our imagination.