Never believe these audience reactions are spontaneous. WWE doesn't leave these things to chance; they orchestrate it by the use of "claques."
Claques are groups of people hired by the organization putting on the show. They are placed in strategic locations in the audience and have cues as to when and what to chant. Yes, people are sheep who want only to follow others, but in a crowd of 15,000 people, it would take more than just coincidence to get them chanting "We all hate you" in such timely fashion.
Claques came to being during Vaudeville, when a whole variety of acts appeared on the stage and the audience often didn't know when to applaud. Claques led the way then.....and they apparently still do.
During Hulk Hogan's induction speech when he went into the WWE Hall of Fame, anyone who thinks the chant: "One more match!" originated spontaneously knows nothing about crowd dynamics and probably still believes in the tooth fairy (and I'm not talking about Dwayne Johnson, either).
It was claques that started the chant against Cena last night.....and you'd better believe he knew it was coming.