Why was Isaiah sawn in two, according to the Ascension of Isaiah?

The Ascension of Isaiah gives two reasons, one human and one unseen. On the human side, a false prophet from Samaria named Belchira accused Isaiah before King Manasseh. He said Isaiah had prophesied that Jerusalem and the cities of Judah would be laid waste and that the king himself would be led away with hooks and iron chains. He also charged Isaiah with claiming to have seen God and lived, against the saying of Moses that no one can see God and live. Manasseh was pleased with the charges and had Isaiah seized.

Behind this, the book says, stood Beliar, who lived in the heart of Manasseh. Beliar and Sammael were furious with Isaiah because of his vision, which had exposed Sammael and made known the going forth of the Beloved from the seventh heaven, his crucifixion and his rising on the third day. Isaiah had already told King Hezekiah that this death awaited him. He was sawn in two with a wooden saw, and the book says he neither cried out nor wept.