From what I've read, the clearances were such that the placement of the moving vehicles under the bridge had to be changed to accommodate obstructions, thereby altering the stresses on the bridge, leading to the cracking of the concrete, and the "warping" of the internal pipe compression mechanism to the point of uselessness, which meant "FAILURE".
A freeper, perhaps you, posted photos of that. The side that held up had a vehicle support say 15’ from the cantelevered end overhang. The failed side it was say 25’ from the end - and as you say - to avoid the extra shoring, etc. That seems like a good place to start.
Also - seeing as it was a cable bridge, but no cables in place yet - would seem another likely critical factor.