The Deluge refers to the rise in world sea levels during 1885 to 1887 that led to the flooding of the continents. The rise in water was slow but inexorable, and while very few lives were lost as a result of it (and most of those due to stubbornness or ignorance), this last of the the Trinity of Sorrows has as profound effect on the people and nations of the world as had the the Grand Cataclysm or the Ashen Death.
Given that later exploration indicated that both the North and South poles had lost a majority of their ice covering (some estimates imagine as much as 80% of the North Pole has melted), the belief is that the Deluge is a result of this melting. Records have been notoriously unreliable since the Grand Cataclysm, but many have hypothesised that the world is getting warmer and this triggered the loss of ice and subsequent Deluge.
Much like the Cataclysm and the Death, we can only speculate and wonder.