I can believe both because people in general have the capacity for reprehensible behavior but tend to agree with you if only because if that happened a couple of other things would have happened. 1. the girl would hurt. 2. there would be a police report. 3. there would have been a huge fight involving other people that witnessed it including the girls parents.
Given the conditions it's much more likely someone slipped on the stairs, that person was over served, the girl fell maybe a step or two into a bunch of other people. Throwing someone down stairs implies an overt act that would have gotten attention.