It appears the use of magic by man can have a significant effect on the oscillation of mana: according to novels, when the shaman of the Ghost Dance carried their ritual, it made mana level rise a lot faster than it did in previous ages, making the insect spirits arrives way earlier, and the Horrors' arrival a potent threat only five decades after the AWakening.
One possibility I'm considering (should it ever come int play during a game) is the level of mana actually was more or less "flat" for several hundred thousands of years. For all that time, it never fell below the level at which dragon would be sleeping, nor did rise at the level at which Horrors would arrive. It's only something like 30,000 years ago that humanity had spread around the globe and was numerous and "culturally evolved" enough to practice magic at a level that would alter the level of mana, bending it into a sinusoidal curve. Maybe just a dozen of shamans gathering to cast a ritual could have been enough to knock the manasphere off balance. From then on, the mana level started oscillating, without reaching the point at which Horrors would come, before felling to the point it threw the dragons into torpor.
It's still odd to have dragons counting ages starting with on they should have slept through. Maybe it was meant to be "the first age of mankind" without any dragons to watch over them. Maybe the dragons even thought it was the end of the world and they would never awaken anew. Maybe the fear make some of the dragons go into a desperate attempt that backfired, and allowed the Horrors in ?
Just thought though. It may fit in, but it's not baked by anything (except for the Ghost Dance part).
