I hate to be picky, but it does not look to me like the girder "fell", but more like it "sagged". Could be that the girder was not properly supported at mid-span or perhaps it failed somehow. Very strange accident.
You are correct. The top flange of the girder should have been laterally braced at several points. The flange apparently was not braced ar all allowing the girder to
twist at midspan, a common mode of failure.
I beams work 1 way. When they are taller verticaly. This 1 appears to have rolled sideways and bent under its own weight. Looks like it wasn't secured verticaly very well and an earthqake/road vibrations via trucks, might have tipped it.