If they're piloting proper ESes, which they do seem to be, then it means the Vessels of Anima are reacting to them. And the Vessels aren't simple man-made objects. They're what people in my universe term Relics of God.

And...about meeting U-DO. *His voice grows melancholy, tinged with loss.* I wouldn't recommend it. My...I knew someone once who was capable of communicating with it. The resulting stress caused her to grow ill and eventually fall into a coma.