I went over to the official forum for that server, and it looks like a very strange server.
They have their very own
Scientology guild.
That was a pretty shitty thing to do, but they organizers of the event provoked it by not getting a Blizzard GM to protect them from attacks - assuming they actually WANTED people from both factions present. Something very much like this happened in another MMO called Dark Age of Camelot, but they got their GMs to help, and it was a sweet, touching event where enemies set aside their fight for a moment of respect.