Even for lore dorks, the cinematic trailer may not be completely clear. The succinct explanation above was inspired by the World of Warcraft forum post, by user Sozu, linked and quoted below (image link mine).
“Verily Gul’dan I see through your duplicity, for you see that hooded orc [http://i.imgur.com/mCdwE0B.png] is my son from the future. He told me that in fact you do not offer us limitless power, but are instead selling us into slavery. For this reason I will not drink this demon blood, and have used this opportunity to lay a trap for your demon master who foolishly left himself vulnerable without his demon armies, and I shall smash my axe into his face as such.”
This comic was inspired by a post about the history of Azeroth’s races at http://forums.worldofwarcraft.
“Thrall: Let us return to our peaceful shamanistic heritage where the only way to earn true honor is in battle and we had last names like Hellscream and Doomhammer and Bladefist!
Orcs: LOK’TAR OGAR
Thrall: Now, Grom, don’t attack those humans. They don’t know we’re here and we can avoid them.
Grom: Or…. no?
Thrall: Dammit Grom, as the easiest and most noticeable reminder of the Old Horde and its bloodthirsty way, it’s things like this that’re going to make me commission a gigantic statue of you in the Barrens and have the Horde all but worship you.”