Bullied

One day, while waiting in line for lunch at school, I noticed an intense argument. Apparently this girl had slapped this guy while waiting in line. The guy was most obviously mad because his face was a dark red and he was yelling quite loudly. I am not too sure why this girl had slapped the guy but I bet he deserved it. I do not know the background and information of this fight so I can not conclude who the actual bully is. Some people might think that it was the guy because he might have done something to the girl to make her slap him or the the bully could be the girl for slapping the guy. From what I saw I want to think that the girl was the bully. What gave her the right to slap this guy?