That's what I figured, but when we ran back to the beginning there were no mobs there.
Did you see Harpy Wraith alive when you went back?
Its a boss at start of Delta. The point of whole GV is to keep that boss alive, and for that reason you kill all mobs waves and bosses. If any mob pass and kills it then run fails and bosses dont spawn. Thats only thing that comes to my mind.