Magician? Seriously? He was about to lecture her on how magic was not a bloody trick but an instinctive ability that one was born with through pureblood heritage, but how did she not know that already? Obviously this was the muggle world he had stumbled into somehow.
But he'd been at war recently, and he was jumpy, paranoid, like a soldier come home who still flinched at any sound of a loud bang on instinct. At her mention of 'abracadabra' he instantly flinched and stepped away from her, aiming his wand at her. It sounded entirely too much like the Avada Kedavrasarcastically, and that in itself was startling. He realized she must just not be aware of how powerful words could be.
"Don't," Draco told her darkly. "Don't mock magic." She was a muggle for gods sake, she had no right!