As I recall he became fixated on the contents of Hunter Biden's laptop, to the extent that he got himself fired from The Intercept over it. Maybe he saw the pictures of naked Biden and became absolutely obsessed with the photo gallery of coke and dong.
Hard to say whether he resigned, or "resigned". And I'm not sure there's a huge difference, practically speaking. By Greenwald's account, it was the former, I think. Either way, he took his marbles and went home. In fairness, I can imagine why, after having spent 7 years of his life mining through the Snowden archive, spending another few years mining the Biden hard disk might have been attractive.