Disclaimer #1, I'm not a structural engineer. That said, this photo along with the security cam footage of the collapse indicate the collapse started on the south side of the bridge which this photo is facing (and if the link isn't working right, it's frame 49, also the description from CNN showing the bridge collapse is wrong, the footage is from the south end of the bridge, closer to downtown, not the north side as they describe it here).

What the photo, combined with the video are making the non-engineer ask is, "See that section on the bank that points towards the south at about a 20 degree angle? That section of bridge did not fall down, like the rest of the bridge, it fell sideways. Why?" I'd start looking there.

35W Bridge collapse by jpiehowski