Each character in the title had a member assigned to it:

But there's only 8 characters (if you include the "!"), so Aika got the special role of Hanasaka Jiisan instead.
Still doesn't answer the question of why Aika was the one picked though.
A random lottery is as good a guess as anything.
Seeing all of them lined up there, I really get the feeling that they were going for a kind of optical illusion kind of thing.
Fuku-chan looks comically huge next to the "su".
Speaking of Fuku-chan:

Seems she's the one jumping the highest during the Musix performance. (Right side)
I can't help but think the fact that the only ones jumping like that are Fuku-chan and Gaki-san isn't a coincidence.
We've seen Gaki overdo dance moves before, so I'm thinking that rubbed off on Fuku-chan since they dance right next to each other.