NTSB are on site and are investigating and we really ought to await their report. Their reports are always excellent, but they take about 2 years to come out - which is an awfully long time. Meanwhile it is difficult to resist the temptation to speculate and be an "armchair forensic engineer".

Wikipedia has already started a page about this bridge collapse. They show it is a steel bridge, but there is no evidence of any paint on the steel. I guess this means it was made of "Cor-Ten" or weathering steel. Weathering steel — Wikipedia Maybe it should have been painted anyway?

The wiki page also highlights a cross-braced support stanchion sporting a couple of skinny bits of wire arranged in an "X" shape. Fern Hollow Bridge pier (2015) — Fern Hollow Bridge — Wikipedia. Probably doing more harm than good because they are probably under tension — thus adding stress to a place where more stress is the last thing you need.

In this Google Streetview Tom Batroney — Google Maps (dated 2020) we see that the lower bracing has disappeared altogether. I doubt if it just fell off. It is more likely that some infrastructure engineering organisation did some calculations which indicated that it was safe to remove the bracing — and now nature has proven them wrong. That party is probably now in considerable trouble.

In your own photo, we can see the left baseplate and nothing else — no mangled remains of stanchion attached to it, as one would normally expect. The stanchion has completely sheared off the baseplate. This too is pretty damning.