Consider Partial Cooperative Storytelling
Many DMs engage their players more deeply in setting the story elements for their adventures. See the article Worlds of “OUR” Imagination and GM-less games like Fiasco and Microscope for inspiration.
Since they know more about the places, history, and people of the world, ask them why they think your crisis is happening, who might be behind it, and why. Just the process of them discussing it should provide enough background to help you refine your view on the threat and its source. Then, between sessions, you can dig deeper with a focus on just the details that have come up.
Honestly, we DMs do this all the time – when the players come up with far better ideas, fears, and rationalization based on partial information than you'd previously worked up (or found in a module), we weave the detail into the story in real-time. If we're not sure of how things are connected, we ask them to flesh it out – and with D&D we get to make it all seem "part of the plan" by asking for detail and skill rolls.
Here's an example:
Player-Who-Knows-More-History: "Hey! I bet this is part of some plot by [old nemesis I'd forgotten about] in his ongoing plan to open an obsidian gate!"
DM (after thinking quickly): "Sure enough, you look closely for his tell-tale signature… Tell me what you're looking for."
Another player: "A red-tipped crossbow bolt!"
DM: (Rolls) "15! … and you find the broken red tip of a crossbow bolt that was overlooked under the body."
That example is very sandbox-ish, but I do it all the time in even the most linear-on-rails modules. The players always seem more engaged when the story goes in their direction, even if only a little bit.
Even in a GM-centric game, it is still "our" story – try to embrace as much player input as you reasonably can.