Well it actually makes sense though. I was thinking Shield Oath wasn't working. In my case my OT switched from Shield to Sword. Therefore he'd continue getting the threat bonus and the DPS bonus making it difficult for me to keep up in threat.

That extra 50dmg a swing makes a difference!

At the end of the day, threat management is important. I watch my threat I want to be near the MT incase he dies but not close enough that one good hit will cause me to overtake him.