That was because of a freak gust of wind. No outdoor stage like that could have stood up to it.
Well, yeah, the wind gave it a push, but the stage wasn't designed/built properly. There was absolutely no strength for a sideways load due to lack of truss triangulation. The entire setup was on vertical stilts with no triangular braces...OSHA and others pitched a fit when they inspected the wreckage...
Second thing, thought State Police and the State Fair people protested, the people in charge (promoters, band, stage manager) decided to continue the show, even though a severe thunderstorm was on the way. Any number of people should have shut the thing down. No one did. (Though supposedly the State Police Captain and the head of the State Fair were headed onstage to make a cancellation/postponement announcement, but they didn't make it in time.)