I'd only seen 1 of the stewards acting over the top, the rest of the stewards involved in the incident only did what was necessary to defuse the situation. I spoke to a few of the stewards afterwards about the supervisors behaviour and they seemed just as disgusted by it as we were.
Didn't see what caused it but the steward (actually was a supervisor) was certainly throwing more punches than the fan. Even after other stewards stepped in and pulled the fan away the "supervisor" went back in to throw a few more.