You haven't been doing anything wrong Katyromick. For one thing, I think DKs experience this problem to a lesser degree as they can switch to more threat/damage-generating attacks (Heart Strike) in lieu of of death strike spam as they out-gear content.

More importantly though, if you haven't noticed the Vengeance issue on tank swaps, I would contend that your other tank probably was either getting very streaky with their Vengeance buildup (a problem in itself, as mentioned in Theck's blog), or they weren't maximizing their dps (more of a personal problem, but it masks the mechanical one).

I remember noticing this problem the most on Ragnaros tank swaps with my paladin trying to taunt/pull off of my bear tank teammate. Big swing damage (or avoidance), the Burning Wound debuff, combined with the bear's ability to stack more agi/stam compared to my paladin favoring mastery put this issue under the microscope for me. Even with preemptive taunting, popping wings, holding a SotR for the taunt, and occasionally hitting the bear with Salv still wouldn't always hold off their threat gains. That's absurd. The difference was so stark that encounters with good Vengeance would allow this bear to be doing 25k dps compared to my ~15k dps. Nothing in my rotation could be changed to compensate.

Theck's solution is very elegant and definitely would make me enjoy tanking more.