Lets say that one of the tension members in the center span (one on the bottom) had a fatigue crack that was ignored or missed. The bumper to bumper traffic and the construction equipment were too heavy for the remaining metal.
The tension member would snap. That side would start to fall straight down. It would pull the other side out of vertical and that would fail before it got very far off vertical. The center section would then fall straight down (although tipped slightly since one side got a head start), pulling the supports on each bank towards the water. That is what it looks like in your photo.
Once those supports were gone, the bridge approach structures on both sides would fall, starting from the missing supports and working in both directions until it reached solid ground (or very close to it).
BTW, the researchers will be looking for a piece that has a crystalline (brittle) crack for about half its width. That part will be rusty because it took YEARS to develop. The rest of the break will be ductile and have little rust (only that which forms from the yesterday to when they pull it out of the water). That will be the initiation point for failure.
Anyway, that is what my money is on.