Well, it's not hard to make things move with sound. I'll bet there was a good amount of wind being generated by the speakers at that concert, and indeed every concert. My home stereo system can blow papers around if I set those papers right in front of the subwoofer. The speakers in my car and blow papers around if they're laying on the floor.

Speakers move air. That's what they do. Making objects move is a simple as them being light enough to blow in the breeze or vibrations created.