Finding a moment when Toph was alone was tricky.
The blind bender, though not one for dress-up and formal gatherings, apparently liked to socialize. For the majority of the party she was glued to Iroh's side, alternating between laughing it up and complaining about Zuko and Katara's choice in partners (at least someone understood this dilemma). When she wasn't with Iroh, she was with Sokka and Suki. When she wasn't with them, she was harassing Aang in the usual friendly manner.
In fact, Toph was never alone – she always found someone to talk to, seeming determined to make the most of an event that forced her into an uncomfortable dress.
So Yue decided to make her move out in the open. When she actually stopped to think about it, it was probably better that way – attracted more attention.
Which is how the great Toph BeiFong collapsed mid-sentence, going cold and limp, her vibrant aura vanishing as it was banished to Koh's lair.
It was horribly cruel, but the only way to get Aang's attention.
Author's Notes: Super-short and evil, I know. But it's for the best, I promise. Not having control over Aang's thoughts forces Yue to take extreme action.
Disclaimer: I do not own 'Avatar' or any of its characters.