Possible causes of Mourning

The Ashbound and the Children of Winter are on the right track: The Mourning was the natural consequence of the extensive use of magic during the Last War. Ending the war has temporarily stopped it, but the Children of Winter believe that the damage cannot be healed: the only way the world can be restored is to go through the winter to reach the spring that lies beyond. If they are correct, the Mourning will eventually spread until it covers the world. But perhaps they’re mistaken, and there’s a way the damage can be undone… but it would still mean that the people of Khorvaire would have to be careful about overuse of magic in the future.
House Cannith was developing a weapon. Something went disastrously wrong. Questions that remain: could this weapon be restored or duplicated? Do any of the current Cannith leaders know about this project? Presuming the forgehold developing it was in Cyre, what happens if the Lord of Blades or someone else discovers it? One of the Overlords of the First Age was bound beneath Cyre. Due to the machinations of the Lords of Dust, the fiend was partially released. The Mourning is a reflection of its influence. At the moment it is building its strength; there is one more step that is required to fully release it. If that occurs, its power – and the Mournland – would spread.
The Lord of Blades was behind the Mourning, an attack targeted against the heart of House Cannith. This may have used an epic artifact or eldritch machine — which could potentially still be tied to an Overlord or to the Daelkyr. Generating the Mourning drained the weapon of power… but the Lord of Blades is working to restore its power. The Mourning was actually caused by dragons of the Chamber, as part of a necessary chain of events to prevent the release of an Overlord — for sake of argument, let’s say Tiamat. The Mourning can be reversed, but reversing it will unleash Tiamat, who will corrupt Argonnessen, and set into motion an epic conflict with the dragons.