Sometimes enemies will attack others regardless of threat; they are programmed to do so. (See 'Trial of the King' in Mogu'Shan palace).
Just because you have the highest threat on a mob doesn't mean it will always attack you, and there's nothing you can do about it. It's how the encounters were designed.
For normal situations TP should never be wrong about aggro.
Please report any Code of Conduct violations, including:
Threats of violence. We take these seriously and will alert the proper authorities.
Posts containing personal information about other players. This includes physical addresses, e-mail addresses, phone numbers, and inappropriate photos and/or videos.
Harassing or discriminatory language. This will not be tolerated.