Massively have coverage of the recent very noticeable mass-death of player characters in Stormwind and Ogrimmar on some servers.
It appears someone was able to spontaneously cause players to die multiple times through some hack. This must have gotten pretty extreme for a scene like the above to occur.
Best quote from the comments that I found was WaitWhut’s
Not even Deathwing managed that.
I won’t jump on the bandwagon of calling Blizzard over coding quality, testing or whatever. They fixed the bug pretty quickly, and to be honest a lot of MMO developers are facing problems like this at present. I would say that smaller scale, less noticeable hacks such as botting is on the rise in most games – Guild Wars 2 has them apparently (though I haven’t seen any on my server, guild mates have) and I’m sure many other games do as well.
Perhaps some talented players were upset there was no player-perpetuated event to launch Mists of Pandaria and decided to get creative and create their own?