For us laymen, what is scour?
Ok, you have a bridge pillar holding up a bridge in the water. If you’re lucky you can get thebase of the pillar or drive pilings down to bedrock. In many case it’s simply too deep and it’s just river muck or sand a long way dowm. So the you have to support base of the pillar with a ton of pillings driven deep and angled, etc. They usually try to then pile “rip rap” (boulders) around the bases of tbe brige pillars underwater.
Bridge supports constrict water flow - especially in floods. You get water just screaming through at high velocity. It will erode away the sand or muck around the base of the bride pillar, undercut it, esp. if you don’t keep checking with divers and adding more boulders and stuff. Eventually, so much sand and mud gets scoured out the pillar is unstable and collapses.
Not saying it happened here but it’s one of a variety of ways a brige can spontaneously collapse.
Scour is when there is, say, a post structure of some kind in the middle of a river, that supports a bridge. As the river water hydraulic forces work on the riverbed around the base of a support, the water force can eventually "scour" out the riverbed around the support structure, eventually undermining the support. At some point the support column collapses and the whole structure comes down.
There have been several bridge collapses from this effect, HOWEVER... this particular bridge does not appear to have had any supports that were actually in the river. There were concrete caisson type supports on either bank but the bridge spanned the entire river with nothing subject to scouring.
So, there's that.