The origin of the Usurpation lies in the Great Curse which caused the Celestial Exalted to be controlled by their passions. With the control of Creation effectively ceded to the Solars after the defeat of the Primordials, the Solar Deliberative became progressively more corrupt as its immortal rulers went progressively more decadent, madder and less controlled. Recognizing that the Solars were becoming progressively more erratic, the Sidereals cast a Great Prophecy predicting the future of Creation and found that there were three possible futures. In one future, the Vizier's would talk with the Solars and they'd listen to reason and things would be well again. In one future, the Sidereals would speak with the Solars but the Solars wouldn't listen and the Solars became more and more corrupt until the world eventually degenerated further into a world of horror and decay; and in the other, the Solars were eliminated and, while the world became a less grand thing than it would be at the apex of the First Age, it would avoid the former fate. The Sidereals took the option to betray the Solars.
The Usurpation was a conspiracy launched by the Sidereals and executed by the Terrestrials who, being creatures of duty, were aghast at the increasing corruption of the Solars (and humiliated by the Solars promoting their own Half-Caste children to unearned positions of rulership). The Terrestrials struck during Calibration, slaughtering most of the Solars at Meru, and then chasing the remainder down. Several Solars fled to hidden Manses to try to outlive their hunters, but the vast majority of Solar Exalted were killed and their Sparks of Exaltation were captured and stored in the Jade Prison.
The Lunars did not participate in the Usurpation, but they didn't stop it either. After the Usurpation began, the majority of Lunars fled to the hinterlands, eventually hiding in the Wyld. Those that didn't die adapted to their new barbaric existence, and many are still tormented by the memory of their betrayals.
With the fall of the Solar Deliberative, the Sidereal Exalted proceeded to fade into the background, instituting the Immaculate Philosophy to raise up the Terrestrials as pawns while they disappeared from history. The Sidereals used Sidereal Astrology to permanently alter the constellation of The Mask and become obscured from history, the Arcane Fate that affects Sidereals dates from this time.