This is a lot of stuff happening very quickly, most of which he can't see or understand. He stumbles backwards, throwing his shield away the moment he notices she's stopped it. He's winding up to try again, to test her weird barrier, and then the sky opens up.

He looks up at each of the portals, his eyes darting from one to the other. He doesn't understand what they are, but for a moment, it feels apocalyptic. It's not until Luka starts yelling at Twelve that he yanks his eyes off the sky. He sees the way the man grabs Eight, and he at least realizes where the portals came from in that moment.

Why Luka sounds so distressed escapes him, of course, but the distress is enough. He retreats backwards, putting himself between Twelve and Luka and setting a shield up between them. He won't say anything here, but he's clearly on edge, his eyes darting from Twelve to Luka and back. If Twelve tries to come at the princess, he will intervene, his whole body tensed to do exactly that.