Looks to me like she meant to do it but misjudged and did it too hard. Like she was going to take her knee and nudge him into the room but didn't realize she was so close to him, so when she brought her knee up it looks like she purposely knees him in the back, knocking him to the ground. To me, it seems obvious that she meant to do it but there just seems like she didn't mean to do it quite as hard as she did. None of her actions before & after kneeing him show anger on her part imo. It would be a different story if her body language conveyed anger or frustration at the kid not going in the room but I don't get any sense of that from the video.