Anyone read Revelation Space? Its the Inhibitors. They woke them. (for those of you who haven't, think halo rings only they are sentient, robotic planets instead of mindless rings of death...)
But in all reality, you have the following options:
1. Nanites: You get enough and via collective computing, you get something that could outsmart you...
2. Genetic Experiment Gone Awry: They intended to create a new subspecies of themselves that was far superior to the originals.
3. Slave Rebellion: By increasing their intelligence to increase production rates, the slaves became too intelligent and bid their time until the Vasari were unprepared and struck.
4. Inhibitors: By discovering the inhibitors, they created something that could destroy them and without Amarantin technology, they could not produce shrouds to protect themselves.
5. Super-Race: Perhaps they stumbled across a gate used by said race and because that race realizes that they are close to creating a weapon that could rival their own, they decide to crush the Vasari.
6. Virus: By using a custom virus to assassinate an official, some scientist accidentally created something that could kill the entire race.
