Thanos isn't just a man, but a spirit. This spirit makes the entire Avengers, and indeed the entire universe, tremble. When the scales of fairness and justice tilt, and no one steps forward to rectify this injustice... then all will ultimately end in destruction. Rory's appearance perfectly disrupts this delicate balance. When three Captain Americas appear before him simultaneously—when two generations of Sorcerer Supremes, the Ancient One and Strange, appear simultaneously—when a thousand Avengers appear simultaneously—Rory knows...he's in deep trouble.