Severus Snape didn't cause commotions. Mary's blatant misunderstanding could not go uncorrected, but Sev didn't want to stand around when he was still in earshot of Ted's multitude of irritating and overly dramatic friends. "I was unfortunate enough to bear witness to the commotion, that hardly means that I was the catalyst," he snapped back as he stalked past her. He didn't want to stay in place and he didn't particularly want company, but if Mary were to follow him, she was likely one of the few people in camp who could do so without getting cursed.
"Potter and Longbottom are what happened. They are so consumed with the need to be big damn heroes that they see darkness and danger when there is none." It wasn't an explanation, Sev didn't feel like he owed Mary that or anything else, but it was something.
Resentment and dislike were radiating from Sev in almost visible waves as he shot a look over his shoulder to confirm that he hadn't been followed. There was no sign of James Potter, but the prick had an infuriating habit of appearing out of thin air, even when strong disillusionment charms were in place. "Ted made a credible go at appearing to be dead, and when they found a potion vial in his hand, they automatically assumed I was either the world's most incompetent potioneer, or a murderer without the sense to cover his tracks."