why else would it collapse? someone below dislodging a support?
I'd handwave this away with vibratory mode not anticipated by the builders. triggered by the sharp impulse from releasing the tension on the handrails—sort of like the amateur version of the Tacoma Narrows bridge failure.
I think it's even simpler that that. I think that the rails were providing almost all of the stiffness against simple flexion of the surface of the bridge under the load of that many people. However, nothing was keeping the rails perpendicular to the bridge's surface apart from the ribbon, and without that restorative force, they buckled immediately and catastrophically.
I recently built a tower out of spaghetti and masking tape that could support a cup of water at the top as a challenge, and the rigidity of the whole thing ended up being dependent on just the tiniest of fixups on the bottom layer. One single snapped strand of spaghetto, and 200 - soon to be 400 - other bits of spaghetti and half a litre of water would have come crashing down. This bridge kinda reminded me of that tower (and which still stands in my lounge, I'm kinda proud of it).
