The great celestial changes occur every 576 years. Basically, over the course of time, void of corruption continues to seep deeper and deeper into the elemental planes, until such time as action is required to cleanse the planes. Thus, once every 576 years, a cleansing process takes place, to wipe out not all, but a large portion of the void of corruption that infects the planes.
This cleaning process, unfortunately, is extremely unstable, and as such can oftentimes have many unintended consequences. We all know that magicks change over time. Spells that used to function no longer do, and spell have been created that were never in existence before. The great celestial changes, and the instability they cause, are the primary reasons for this.
However, the last great celestial change, of 591, is different from all of the others before it. Magic tended to stabilize itself after previous celestial shifts. After the celestial change of 591, magic never stabilized. We have been losing spells, gaining spells, and having spells change their potency all over the place. For example, compare today to the mid-590's. A cure light wounds spell is more than twice as effective now as it was then. A magic missile, however, is only slightly more effective now than it was then. Even stranger, spells now require numbers added to the end of them in order to function properly, which was never necessary in older incants.
How do we explain this constant magical flux? Well, the researchers who wrote the Tyrran Cosmology believe that there is a great magical change coming, one that is bigger than all the others before it, and that this magical change will be caused by the eight immaterial planes being released from the grip of time, and going back to being independent. Such a change would cause amazing magical fluxes, like nothing we have ever seen before, and will allow the universe to go back to what it was before the First Elemental War. The researchers have named this coming change The Great Upheaval.