I'm guessing that this is something they do if the previous emperor dies of natural causes or willingly abdicates.

I can't remember how many candidates they said there were, but I assume each one is sponsored by some kind of nobility within the Empire.

With both The Hunger Games and the Terrans being inspired by the Roman Empire, I didn't think it was entirely out of left field (with the caveat that the Mirror Universe is a very silly place).