They NEVER got free.
They're still imprisoned, Ulduar is a PRISON.
Suddenly ? Nope, the titan watchers got corrupted after years and years of corruption by Yogg's whisps. It's the same for other old gods, we only noticed what they're doing after years and years of subtle corruption.
And nothing says that Hakkar is a minion of the old gods, that's a speculation.
But they got created by the Old Gods, they're a part of their power, for exemple we wouldn't give warlock's credits to their demons.