From my view of the bridge from various Fox reports.....
The main span collapsed. It balanced the spans on each end and loss of the balancing load caused the end spans to collapse in the opposite direction.
The main span appears to be supportes at the ends by a steel truss. Failure of one or more of the truss connections would result on progressive overstress of other connections and collapse.
Which joint failed? What caused the very localized initial joint failure.
Sabotage of the critical joint should be suspect.
Bert’s off the cuff analysis.
I theory I’ve had is that the bearing concrete pylon and footing settled — that is the footing slipped — moved deeper or sideways into the its bedding. That created a torque at the one bearing point, and structure does not handle such torques well — the joints started popping, catastrophic collapse followed.