He was probably doing Fast Blade > Savage Blade > Provoke > Rage of Halone. That comboed RoH has an incredible amount of enmity on it, and it's landing right after the Provoke shifts him to the top of the enmity list. It's a great way to quickly handle a tank swap mechanic, or to troll the ever-living hell out of your entire alliance... >.<

As to how he was HOLDING aggro against you, that I can't really be sure on.