Somewhat disagree. Every bridge I am aware of (I used to inspect them and supervised other inspectors afterward) has bad expansion joints. Every one of them is rusted and most are totally locked up. It sometimes causes localized failures, particularly on the deck if it is concrete, however, I have never heard of that being a primary cause of failure.
My bet is a fatigue crack that was hidden or missed during inspection. The combination of bumper-to-bumper traffic and the construction machinery was more than the remaining tension (the uncracked area) could take.
Over at
there is a good diagram.
The center span over water is 468’ (if I recall correctly) of the 1900’ total.
The whole bridge was limited to one lane each direction where there is normally four lanes of heavy traffic each direction.