In the Xenogenisis series by Octavia Butler (which I highly recommend even though I'm only middle of book 2), the alien race the Oankali show up just in time to "rescue" humanity, who have destroyed most of Earth and human fertility in a nuclear war.
How did the Oankali happen to time it that way? Were they tracking human progress from afar and only came to Earth when humans went to the brink of destruction? Did they somehow trigger the war so they could set humanity up to need rescue? Was it just an astronomical coincidence? Something else?
Related thought is -- the Oankali continually justify their trading with humans and the injustices therein by pointing out that the humans had nearly destroyed themselves. So... is that also the case with other species they trade with? And if so, again it begs the question of how they can know when a trade-worthy species is at the point of needing rescue/trade?