[Rezz]

Considering the war with the Scourge is still going on the entire time, it's not out of the realm of possibility that since landing on Azeroth some of the draenei have been "turned."

They could easily spin it as part of the wreckage from the Exodar crashed near Icecrown and Arthas did whatever funkified magickz on em to make em Death Knights. I mean, his power is much stronger than the original Lich King's, so his ability to turn people to death knights is probably amplified. Would explain why the Lich King originally needed to corrupt people first, maybe Arthas just captures, mind-melds or something and then trains them in the evil ways.

Who knows, they better have awesome fucking armor though.

edit - as to them treading on other tanks, according to the interview with Kaplan and the other dude on Curse that probably won't be the case. They wield a 2hander, thus they have no block rating (has there been any word on them even being able to equip shields?) so 45% increased armor is probably to counterbalance the lack of a shield. They will most likely have slightly higher parry rates but still be physically weaker at taking blows than both druids and paladins, not to mention warriors. Druids will still be the kings of non-crushing slow dps, paladins the AE tankers and Warriors the single target tankers. They're just removing the necessity of warlock/mage tanks for any fights by adding in a class that deals more with the anti-caster role. Leothras the Blind and Solarian come to mind.