I think what drove Damian to hunt Deathstroke is more than he irritated because Deathstroke ruffle his hair.
See, Damian bleeds for acceptance. That's his whole character. To impress his mother, to impress his father, to impress Dick, he'll do everything to get a simplest 'good job kid'. So every recognition gesture from other is precious to him. Now comes the 'ruffle' problem. I think he's angry because he LIKES it. Deathstroke gave him a simple affection gesture and he's so starved for that , because Bruce is emotionally constipated, Talia is Talia and Dick is Ric, so he can't bring himself to not like it. And that's disgust him. Especially because Slade was once rumored to be Damian's father and we know how Damian hates the thought of it but little part of him can't help but doubt, maybe he's really Deathstroke's son.
Damian is 13, he's emotionally unstable. Just like KF said, the 'ruffle' is not the main problem, but it's certainly the trigger. That ruffle made him to feel angry, happy, disgust, shame, sadness, and they were mixed and clashed in his heart. And KF ask, who is Damian angry to? The problem is, Damian doesn't know. Is he angry to his mother, to his father, to himself or to Slade, he himself doesn't know. All he knows that between four of them, Slade is the most punchable , so punch Slade he did. Haha.. Bruce's son..