Terrible thought: what if we already knew what (the avatar of) Azeroth looked like?
It could be that she had subconsciously created a quasi-mortal image of herself with limited memories but the Titanic affinity for the Arcane, perfect but flawed, born to mortal members of an aspiring race, entrancing them and becoming their queen.
When she came into contact with another Titan, obviously this was the only being, to her, worthy enough to be her partner.
Eventually though, this avatar was corrupted by the Old Gods, which may explain why they can be so much more active than before the Sundering.