She elbowed him to the face first.
Oh wonderful, your usual forensic screencap analysis. I think I see Holtby in the corner smacking them both around with his hair.
Watch the video closely. Her arm seems to be extended in order to press the elevator buttons. Her elbow flies straight up like it's been pulled or she's been smacked from below. Her shoulder doesn't move like it would if you were delivering an elbow, nor does the rest of her body.
The video is also choppy. There are dropped frames or missing parts of the sequence. At the moment when her arm is up, Rice's left hand disappears for a single frame, then it's back. The TMZ watermark makes it hard to see. But you can see Rice's left shoulder move upward and inward as though he made a quick move to smack or grab her arm. She could have been pulling it out of his grasp after the initial move and it would have been too fast for the frame rate. Also note that Rice's head doesn't seem to snap back from any elbow to the face. So even if she landed a blow it wasn't much.
My guess is he didn't appreciate the little sissy backhand slap she gave him in the hallway and he figured he'd teach her a lesson as soon as the doors closed. He grabs her to control her, she resists and pulls away and gets slapped. She charges and he punches her.
Much more plausible than an unnatural looking chickenwing elbow to the face out of nowhere.