One possibility is that there was a potential difference between the building/municipal ground electrode and the utility-maintained {pole/pad} ground electrode—causing a voltage gradient leading to the energized enclosures. If any of the sound gear was affected, maybe some sort of under/overvoltage trip/disconnect device in your AC-distribution box would be a wise investment.
I have an long-time associate that used to do commercial sound reinforcement, and before he moved on he’d seriously considered powering all his equipment full-time from his own genset in the alley. Apparently his experience with some public-building electrical systems was not all that wonderful. As for when—in his time the light-show crew was always taking out breakers and fuses at various levels
