cool things with wheels since 2006
It's a combination of both. Those trailers are incredibly light to increase payload for a given max GVWR and those steel coils are very heavy. That's why when you'll only see one being hauled at a time while there is room for several more. Poor driving and rust may have contributed also. Look at the photo. The trailer appears to have a steel frame. The driver may have cut the corner too hard, at too high of a speed. If the wheels ran over the curb, then slammed down onto the street, that may have been enough to cause the trailer to fail if the frame had been corroded by road salt.
That was the same thought I had when I saw the picture, that he'd hit a bump or something to set it off.