He had just finished throwing the ball. Most defenders will go up to tip the pass, but Harrison aimed for a headshot.
The only thing that makes this a certainty for me is that McCoy was standing perfectly upright. In that case, he was not preparing for contact, and crouching down as RBs do. And somehow, Harrison's shot lands smack in the middle of his face mask?
The guy was not in a position to protect himself after just throwing the ball. I don't see the issue calling him defenseless, as he pretty much gave Harrison every opportunity to get him in the mid-section, but Harrison still went for the head. Long story short, had Harrison leveled him with a wrap tackle through the midsection, it would've been fine with no penalty, fine, or suspension. Instead, he did what he intended to do, and got the punishment that he should have.