Uplift/WMG
The Buyur have been planning to create a new Galactic society in Galaxy 2.
By the end of the Uplift Storm trilogy, the great Sages of Jijo have figured out that they were "summoned" to the Slope by the psi-active stone and especially the Great Egg. Something was encouraging sooner races to settle on this one planet (conveniently located next to Izmunuti, in a galaxy conveniently empty of other oxygen-breathers), on this one specific sub-continent. And the rewqs and Great Egg were just the right stimuli to bring world peace and cooperation to the Six. The Glavers just happened to pick this spot to devolve into presapience, bringing the religion of devolving to the other sooners, to repay some obscure promise to their grand-patrons, the Buyur. Now Galaxy 2 is cut off from the others, and will never be re-settled by other oxygen-breathers, so the Six can found a new galactic society however they want. Coincidence?
Hardly. We know from the short story "Temptation" that somebody is living underground, fostering a society of "wizards" who can command swarms of small creatures with magic words, and are using mind control and amnesia tricks to recruit new members of their little enclaves. Except magic doesn't exist: those swarms of small creatures must be genetically engineered to respond to specific "magic" words. We also know that the Buyur, who once owned Jijo, were known for two things: a peculiar sense of humor, and a tendency to genetically engineer small animals into useful tools, such as clock teets and privacy wasps. Such as rewqs. We also know that anyone iconoclastic enough to reject the prevailing mathematical paradigms could discover that Galaxy 2 was going to split off, and indeed some highly-ranked races must be aware of the galactic splitting because why else would they evacuate an entire galaxy?
So: The Buyur knew, somehow, that Galaxy 2 would eventually split from the other galaxies, and that it would be evacuated of oxygen-breathers before then. So they found the perfect planet, with a geology prone to psi-active stone and a convenient carbon-soot star right next to the nearest transfer-point. They petitioned for colonization rights when Jijo became available. They put the Glavers in debt to them (not hard to do to one's grand-clients) and required them to "pay it back" by coming to Jijo and devolving into presapients. And they gave the Glavers a set of Scrolls for their devolving religion custom-made to appeal to illegal sooner races. Then, when it came time to evacuate Jijo, they left a secret Buyur colony hiding under the Slope, waiting. They stocked their colony with whatever technology they'd need to survive indefinitely, and to start a brand-new galactic civilization when the time was right. They manipulated the psi-active flows of stone to entice sooner races, and let Izmunuti and the Scrolls do the rest. Soon they'd have an "empty" galaxy all to themselves, and a planet stocked with an improbably large number of presapient races, shorn of all memory of galactic culture and history, ready for uplift by their new Buyur patrons, ready to swallow whatever revisionist history and novel society the Buyur invent.
The Buyur must have sent the Egg, and created the rewqs, to prevent their future clients from wiping each other out with war, and to encourage the religion that grew up around the Scrolls. They created the underground "wizard" enclaves revealed in "Temptation," based on human superstitions (learned from abductees? from stolen books?), as another experiment in controlling the sooner races. They will, with whatever civilization-restarting technology they've got squirreled away down there, drive off the Jophur who threaten their colony. They'll make sure the sooners think they're doing all the work (with help from the newly "awakened" Egg) and that they bond more closely over the great difficulties, but the outcome won't be in doubt. When the Jophur are "finally" defeated, the Buyur will still be there, waiting, manipulating.
The Progenitors are the closest thing humanity has to patrons
They wanted to see if it was possible for another species to evolve sapience on its own. So they left Earth out of the Library in order to keep their assorted descendants from finding early hominids and interfering with their evolution.
- Alternatively, someone else left Earth fallow as an experiment to see if this would happen but are either no longer around; forgot about it; or were unprepared for the political fallout when their experiment was successful and are now reticent to own up
Noors caused the Great Fire at the Biblos Archive
The sentient noors don't want to let the other sooner species on Jijo find out they're sapient. They managed to keep their secret mostly intact during the earlier settlements, since the first races gave up writing immediately and forgot most details about Galactic species. But the humans had to go print a bunch of books, including a Xenology archive with info and pictures of many species. As soon as the noors found out about it, they went and burned down the Xenology section of Biblos, so nobody would realize they're all tytlal.
Noors caused every unexplained disaster in Jijoan history
They all have the temperament of cats, and an unknown fraction are also sapient tytlal tricksters. Nobody ever suspects them of anything because everybody assumes they're just animals. There's no way to prove they didn't cause any given unexplained catastrophe, is there?