How was that possible when the imprinter only wants to make the imprintee happy? What could have possibly caused Sam to get angry in the presence of Emily?
Reading Sam and Emily's bios answers the why for this, but the how it was possible is, I think, just as simple as she was in the physical space that Sam exploded into as a werewolf. I don't think it was that Sam did it against his imprinted nature, so to speak...it was that he couldn't stop himself from phasing and was trying not to hurt Emily---he was trying to warn her off, but she stepped forward and boom! the werewolf's claw came down exactly on her face. I don't think he reached out to claw her. It's like if you're standing next to a hot wheel car. You're holding it right in your hand or staring right at its grill. Then, all of a sudden, it explodes into a horse-sized hot wheel car. Now it's grill is ON your face.
Anyway, all that to say that I don't it was intentional---more an issue of space.