Bass Frequencies, part of sound check, it just happen to be the harmonic frequency of the walls!
yup, the sound guy tested them so they went to the edge but no more.
Calculated to add visceral impact to the tune.
It wouldn't have been as effective if it were like that the whole night.
The dance floor may not be as bouncy as before but I think you can get a good rattle out of the walls if you try. LOL