Steel rolls roll off truck bed - GIF

[Crusty]

That coil's position may be due to what's referred to as bridge weight. The front load axles and the rear axles should be carrying equal weight (for an equal number of wheels) so that the wheel loading at each end of the "bridge" is within legal limits for that span. Too much weight concentrated on too few wheels can damage the roadway and the trucker's next mortgage payment when the DOT discovers it. Looking at the situation I'd guess that there was a shock from the railroad tracks that momentarily overstressed the trailer because it could have been going too fast for that bump.