backstory is simple: Rob was brought in as adult supervision in case Andrew Mason couldn't handle his shit. Everyone was worried about this with Andrew early on (this is why so many VCs passed on Groupon in '09). Now Andrew has consolidated his position and at least proven he can handle rapid growth. No need for Rob. Therefore, Rob (who doesn't like Chicago anyway) has no reason to stay because he's not going to be CEO.
That's all the backstory you need.