i'm not sure how to explain, since it was obvious to me since the first time I heard the song, but I'll try.
You know when like a bright camera flashes at you that you don't expect, you kind stumble back all surprised/shocked and all. Its like that kind of feeling. You know "Boom (there one), Boom (and another over there?) Boom (and even more behind me?)," so it endless bright cameras flashing in your face and you have no where to go. The Boom is more the feeling of shock with it happening very very rapidly. Its just to give the feeling.
Its a pop song, so don't over analyze it too much.
Is it just me or are they not as sharp/graceful/in sync as usual? It looks like they didn't have enough time to polish this routine before they had to shoot it.