I don't know how long you've been out of school, but it hasn't been that long since I've sat in one of those desks. And those things are amazingly cumbersome. Since the desk goes over your legs, you can't just stand and be out of it. You have to shimmy your way in and out of the chair. The officer wasn't being overly aggressive, the desk got in the way. When I watched the video, I heard him calmly and quietly say "I'm gonna try to get you up now," and when he reached down for her, she struggled. What you have is a person fighting to stay in the chair as the officer tries to get her up in a chair that restricts movement and easily getting up.
Because the desk was in the way, it wasn't as simply as just lifting her up or standing straight up, he had to move her out of the path of the desk. Because she was struggling, this caused an unbalance in the chair, as he has to move her to the side and then upwards. The balance caused the chair to fall over.
People who say he lifted and then slammed her are wrong. Because the desk of the chair prevents a smooth movement, the struggle naturally caused the chair to spill over. And the desk didn't just disappear when they spilled over either, he still had to move her to the side and then outwards from the desk first, and her resistance caused the motion necessary to do so to be a yanking motion.
This was a not an officer bodyslamming or throwing a girl, this was someone resisting in a chair that greatly restricted movement. This literally would not have happened if she had complied and stood up normally, or simply not resist.