If that photo is anything to go by it looks like the concrete beams forming the centre span were not supported by the horizontal I-beams, causing the span to fa

If that photo is anything to go by it looks like the concrete beams forming the centre span were not supported by the horizontal I-beams, causing the span to fall into the river below. I'll have to assume the centre span originally did rest on those I-beams so it would seem that they have moved, either during construction or due to some external influence.