I personally think they chose Yasuo simply for the Malphite Yasuo combo and that's it, but I could be wrong.
With some of the augments, you can see the synergy. Sona can make her ult cast all around her, Malphite's ult pushes people into an area like a Shyvana ult with his signature knockup at the end, Yasuo can keep ulting on different knocked up targets to keep people CC'd until Malphite's ult is up again, Ziggs can send multiple ults to a target destination, and Jinx can stack up multiple ults and stack up her passive when people use an ult. With all that, you can see how Sona into Malphite into Yasuo (into Malph and Yas repeated) into Ziggs and Jinx makes for a perfect combo for AOE damage with just their ult interactions.
Graves could've worked, but he doesn't offer any utility to his team. Yasuo at least has a knockup and can chain the knockup from Malphite (and Ziggs) and offers a windwall to block important projectiles for your team, where all Graves offers is just more damage and a nearsight and slow, which really doesn't do much considering most of the enemies appear to be casters (Vel'koz) or melee champs (Kha, Skarner, Kayn), and they are bots so they know where you are anyway.