Modern theory holds that the book of Deuteronomy was written by an underground group of Jews who wanted to reform Judaism. This is based on a section of Kings II, wherein King Josiah discovers a "book of the law" in the Temple, and starts instituting reforms based on its contents.
A 19th century German scholar named Dewette noticed that these reforms are strikingly similar to the contents of Deuteronomy. He also noticed that Deuteronomy's language was different from that of the other books, indicating that Deuteronomy may have been authored separately (see the Graf Wellhausen Hypothesis). It is therefore believed that Deuteronomy was the book found by King Josiah in 622 BCE. He read it and proclaimed to the Jewish people that their entire observance of Judaism had been misguided. He instituted a series of reforms based on Deuteronomy, which later was attached to the body of written tradition as the fifth book of the Torah.