[S2 Spoilers] Theory: Caitlyn's already connected all the dots.
In Caitlyn's final scene, we are left with the major cliffhanger of season two's first act: Piltover has declared martial law and Caitlyn has absolute authority. A surprise to everyone, including Caitlyn herself.
This has left many people thinking that Caitlyn, in the throes of her emotional and moral turmoil, will become a dictator. Someone who Ambessa will be able to manipulate with promises of justice for the fallen. But those people forget one thing, Caitlyn isn't just a sharpshooter; she's also a genius. As soon as Ambessa declared her intent to "crown" Caitlyn, the gears started turning.
Caitlyn and the council knew that the attack on the memorial would never have succeeded, not without someone on the inside. Most people probably thought it was another official, similar to Sheriff Marcus, but Cait figured out that it was Ambessa, by proxy, who literally invited the enemy in.
Despite that treachery, we must remember that Caitlyn and co. handled the situation well. Vi and Jayce had defeated Renni, Loris, Steb and Maddie were holding their own against the Berserkers.
It was only the Chemtech Dreadnought who would be a challenge, and the show strongly implied that they were about to defeat the monster.
But then Ambessa and her Noxian contingent were able to swoop in to save the day. Claiming all the glory and painting themselves as a stabilizing force in the process. What perfect timing! Almost like they'd been waiting for the chance to step in...
Caitlyn sees Ambessa kill Renni here, but at the time, it just looked like she was cleaning up the battlefield of an enemy survivor. Except for the fact that it's the only time they do that, the only time the Noxians try to make sure that everyone they kill is certainly dead. For what reason would that be unless Renni was the only one they needed to be dead for sure?
The scene tried to make it look like Ambessa was just remembering what she'd done to bring Piltover to the point where it would elect an autocratic leader, but why would she remember Renni being given the invitation when she wasn't there? It makes more sense for the image to represent Cait connecting the dots. That's why she glares at Ambessa before stepping forward.
The show seems to reinforce this, because Ambessa's eyes are shown to be at equal level with Caitlyn's, when she would ordinarily tower over them.
You could argue that if Caitlyn were aware of Ambessa's scheme, she would have refused the office of Commander, but she really couldn't. The fact of the matter is that Piltover was rudderless and leaderless, on the verge of collapsing in on itself unless someone could take charge. Mel wasn't there, Shoola was too scared to take the reigns, and Salo was flawed in every way possible.
Best case scenario, she turns it down and Piltover returns to its original state, panick-stricken state. Worst case, she turns it down and someone else volunteers to take on the "burden" of leadership, which would lead to debate and division in a time they could not afford it.
Caitlyn was the perfect candidate, the only one whom everyone could agree to give power. The only one good enough to make people forget to question whether anyone should be given that power to begin with.
So, Caitlyn had to step up and fill the role. Ambessa had played the game perfectly, driving Piltover to the brink. Now, all that's left for the newly minted commander is to ride the changes and see how much of the city she can hold together.
