Defective parts or defective design? More stress on the bolts than the engineer predicted?
Can be both. However design of bolt- or rivet-connected bridge spans is a very well known process. Bridges of this type were built for more than a hundred years, entirely without computers; all you need is printed tables. Steel made in China, on the other hand, with no quality control and no incoming inspection, is a very likely culprit. An engineer doesn't claim that "the steel has too much hydrogen" until he has the lab test results in hand.
A comment above mentions that some bolts were loose. If the holes are too large this can result in uneven loading of bolts - and then indeed some will be sheared off, and then the rest follows. If the bolts are not tight then they will experience axial stress instead of shear stress, and the thread will be torn off. It's all very obvious when you look at the bolt, and the guys who are investigating are usually experienced troubleshooters.