Some opponents of the Documentary Hypothesis of the Bible like to say that the hypothesis is only supported because such people make the assumption that the Bible was not written by God and therefore they look for human sources. But really now, allow me to demonstrate why not assuming the Bible to divinely written is not a crazy assumption made by people with an agenda.
Suppose you hand the Bible to a guy in the middle of Africa who has never heard of Judaism or Monotheism of any kind. He is completely free of preconceived notions towards the Bible. He starts reading it. You think he's going to jump up and start screaming that he knows he's reading the word of God? I doubt that. It's just a book to him with some nice stories.
Unless they were socialized into that belief from childhood, it takes convincing to make someone believe that the Bible is divine.
It is a lack of assumptions from which the Documentary Hypothesis springs. I see a book here, I know people write books. Do I know if God is real, do I know if God writes books? No, I don't. The fewest assumptions lies with not assuming the book to be divine.
As an abductive scientific hypothesis it is made with the fewest assumptions. Those who created it are not assuming that it is not divine, they simply do not have any compelling evidence to convince them that it is. And they have a better explanation for the book with all its various textual oddities for which the divine origin argument has difficulty explaining. Weird textual anomalies are expected in the Documentary Hypothesis but need explaining with divine writing.