I would avoid too strong of a twist here, because it can undermine the feel of the entire setting -- if it turns out the Day of Mourning was some kind of divine hoax, for instance, that's probably going to frustrate a lot of players.
To the best of my knowledge, popular Internet theories include:
- Cyre is in Ravenloft (I like Jadasc's version of this -- emphasizing the "perpetual WW1 battlefield" angle really makes it much more evocative).
- Cyre's artificers went too far in pursuit of the ultimate weapon.
- The machinations of one of the setting's big bad guys caused it -- this would be the Inspired, the Lords of Dust, or maybe the lich-queen Vol.
- The Mourning is the Turning of the Age or the result of a somehow failed Turning.
An actual play example
A few years back, I ran a campaign revolving around the Day of Mourning. Here's what we did with it:
The PCs started as Cyran soldiers (several years pre-Mourning, with the players having fore-knowledge that this would be a game about that). Through their heroics, they basically became commandos and then secret agents, working directly for Cyre's chief spymaster. He sent them on various missions, some of which were, of course, related to magical weapons research. They were out on such a mission when the Day of Mourning happened. Of course, the team immediately set upon REVENGE!
Well, after a long search that left them all kinda ragged and corrupted (one of our goals was to have a campaign where characters could be "evil" without doing stage-villain stuff all over -- so rage and selling your soul for power became kind of a thing), they finally discovered that the cause of the Mourning was a Cannith forge deep within Cyre, which had somehow been turned into a device to suck up life, magic, and souls. And, of course, the former boss had orchestrated this. Because he was actually a Lord of Dust, naturally.
I freely admit that this wasn't the most inspiring or surprising choice for a cause, but it worked because it was a suitable explanation for who would be twisted and powerful enough to attempt an atrocity like this, and sufficiently personal that it seemed like a natural part of the character's story. Importantly, it was suitably on-theme and suitably obvious.
The fun twist came at the end of the campaign, where, after defeating him, one of the surviving PCs, a Warforged, took the receptacle of Cyre's stolen souls and returned with it to the secret forge at the heart of the Mournland -- thereby, of course, becoming the Lord of Blades.