[Celjabba]

Perhaps a swarm of asteroid impacts, in increasing waves ? The first wave 15 years ago was small, a big impactor had to be deflected, causing a lot of money to be injected in the space programs . The second wave was bigger than expected and an "almost as big as a dino killer" slipped through the defence. The third wave will hit in 15 years, with hundreds of city-killer impacts scheduled, and a couple big ones. evacuation is decided. Earth is pelted again. And again 15 and 30 years later, by smaller 4th and 5th waves. Observation said there are no more coming, time to resettle.
Of course, what (or who) fired a cosmic shotgun at earth is an interesting question.