These are often caused by dozens of people dancing and stomping in unison. It causes what I think is called a harmonic convergence which weakens the deck and causes a collapse.
My dad led his marine platoon over a suspension foot bridge over a gorge in Ithaca, NY back in WW II. He said it was great fun to march in step and set the bridge to swaying! He fortunately lived to tell about it and fathered me in 1950.
my guess is that some floor beams had too long of an unsupported span that were never designed to carry that much weight.