I'd always assumed they spent a large amount of their time sifting through Fake Leaks from well-meaning cranks, internet trolls and various agencies trying to poison the well, looking for stuff they could corroborate to some degree.

I didn't actually check the timelines, so I could be well wide of the mark here, but had assumed these leaks were published after the NYT article about the frequency of North Korean missile failures went out too, which would lend a degree of public legitimacy to the leak.

Conspiracy theory: if you have a source that takes a monopoly on leaks about agencies X, Y, and Z, you can ignore other submissions. They will give you something every once in a while to retain relevance and when the source wants something specific published, they just do so.