The crowd was waiting for Green Day to come out and perform when they played Bohemian Rhapsody on the loudspeakers to fill in the wait. So the crowd started singing along.
It’s a nice video. Something about everyone singing together, and to this song. Just very nice. PS Reader Mirror found a close up video, from the middle of the crowd, that’s really neat too, since you can see everyone singing – that’s the second video below: