It's not too clear what happens next, honestly. Mr. Lahey isn't supposed to be that terrifying; he's just a guy in glasses, after all. But his heart bottoms out into his shoes as the man, or the apparition of the man, suddenly lunges forward.

Lahey's face is purple with rage, and his arms are outstretched and aimed at Scott's throat. He's trying to choke him!

"NO!"

Stiles is the one who stumbles forward, trying to push Scott out of the way. Here, they're both just human, no werewolf powers to give Scott the edge. And he's not going to let his best friend, his brother, get hurt, not if he can help it. So he's the one that ends up partially deflecting one of those hands. It has the unfortunate side effect of knocking him to the side, the smack that landed against his jaw ringing his bell for a moment.