I'm no engineer. But my money is on asymmetric load stressing due to all but one lane being closed for repairs combined with dynamic aeroelasticity flutter resulting in a destructive resonance which shook the bridge apart.
This sort of thing scares the shit out of me.
Bridges are designed by professional, college educated, civil engineers but maintained by those 5/1 city workers (1 guy working, 5 guys watching) union yokels with their propensity for using steel plates to mark their territory.
They don't know shit about harmonics, resonance, stress or balance.
I could be wrong.
I probably am. I'm seldom as smart as I think I am.
At least that's what all of my wives told me. Over, and over, and over again. Then once more just for "emphasis".