At John Cena's request, "wild boy" Machine Gun Kelly performed his single Invincible to introduce Cena to the sold out crowd of 80,000 hyper wrestling fans.  P. Diddy first appeared on the stage to introduce MGK, who was accompanied by Skylar Grey.  

The pair kicked off their song, which set the tone for the upcoming main event featuring The Rock vs John Cena.  The song is really uplifting, so maybe it wasn't the actual song, or MGK, or Skylar Grey that caused a massive uproar of boos from the crowd that could be heard over the cheers.

The match took place in The Rock's hometown, so it was probably the fact that MGK showed love to John Cena after his performance, and the crowd decided it was only right to boo him for supporting Cena.  

Still, Invincible is a great song.

Source: missinfo.tv