The reason is believed to be because we R'Thakians, at that time, ascended. However, i've always been confused at the idea that we are actually ascended, because it's not reasonable that my other half is. We both agree to this. Likewise, R'Thak as we know it is and was not an iceball. Then, or sense then.
Well, my hypothesis is that with rogue body, the planet was knocked out of orbit. R'Thakians have never had true space flight or really entertained thoughts, as a society, about interplanetary colonization. This is due to multiple factors, but suffice it to say, they were basically trapped on a dying planet. I believe they might have pulled their collective experiences, and did exactly what we R'Thakians are taught not to do: use forbidden magics and alter our course of destiny.
In other words, the only way to leave the planet and survive was to... leave. Reality, that is. They basically pulled the entire populous all at once into a high plane of being. From here my best guess is that they either corrected the orbit and then re-incorporated, effectively "descending". Alternatively, they did this by choosing a different worldline with a R'Thak that never evolved sapient species. (This is one of my alternative theories as to why my other half is both in my future, as well as aside me temporally.)
This all is important because some of those clues in the story point to this rogue body coming with a change of the skys. I believe they meant "stars", a "change in the stars". This points to a merging of the Milky Way and Andromeda galaxies. The merging is not expected to have a serious chance of any particular gravity well incepting another, currently on earth. But... what if that near-zero chance is exactly what happened to R'Thak?
That would put my other half's legend time line at around four billions from where we see things.