I'm assuming DK taunt mechanics work like other classes. When you taunt, it puts you at the top of the aggro list, but you have to out-threat the person you taunted off of by normal aggro-pulling range (I think 30%?) before taunt wears off. It's possible that the pally was still outputting a lot of threat, and you weren't able to out-threat him. That'd be my best guess as to what happened.