He did not mean to - it was completely involuntary - but right after Grayson spoke, Damian stole a quick glance to see how his mother reacted. Right after he caught himself doing that, he stiffened his spine and stared mutinously at Grayson.
"Who died and made you the boss of me?" His voice had a quiet challenge in it, but it was covered up with childish petulance. His body language practically screamed defiance. He shifted his weight more evenly, ready to move if need be. He crossed his arms over his chest, tilted his head slightly to the side and then back in an overtly arrogant display. Damian was an old hand at willful disobedience, at fighting for no other reason than fight.
Talia stepped closer to Damian, reaching out to rest her hands possessively on Damian's shoulders. He twitched slightly at the contact, disliking the feeling of someone touching him. His instincts told him that it was an attack, because no one really ever touched him otherwise. She challenged Nightwing too, echoing Damian's words but added new meanings that he had no intended, "Are you going to force him to leave me?" Damian had not thought about whether or not he wanted to stay or go, he simply resented the implication that he would go where ever Grayson told him to.