In Eberron there is an event known as the Day of Mourning, that changed the face of the world forever, and feeds the political fears and motivations of many characters within the system. I know no strictly canonical answer for the cause has ever been presented, but several possibilities have been said over the years both in official and unofficial capacity (I know the last issue of Dragon Magazine addressed it).
Many solutions have become quite popular (magic spell, warforged factory explosion). However, in Eberron, secrets get out, and players become informed. What off the beaten path explanations for the Mourning can I use that players may not expect, and will lead to an enjoyable game?