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.”