One reason could be jealousy. Maybe the girl on the desk had something the girl in hoodie wanted, like popularity or a special talent. So, out of envy, she knocked her off the desk.
It could be a case of self - defense gone wrong. Perhaps the girl on the desk was about to attack the girl in hoodie first, and the girl in hoodie overreacted and pushed her off the desk instead of just fending her off.
There might be a long - standing feud between them. They could have been enemies for a while due to some past incidents. And this incident of knocking the girl off the desk was just another escalation in their ongoing conflict. For example, if they had been competing for a role in a school play and the girl on the desk got the role, the girl in hoodie might have been seething with anger and finally snapped.