Why do they only meet ***** when they go to *******?

It's only when people go to the tunnel at the very bottom of the silo that the AI voice starts talking to them directly, and tells them about the safeguard, but why there?

Three of the people already had access to the AI in the vault while IT shadows, so it's not the first time they've interacted with it. What is the significance of finding the tunnel and that being why the AI decides to give it's plan away?