In the first video, at about 1:22, you can see the structure looks like it has moved a bit prior to collapse. Possibly scour underneath the further away pier, especially given the way the entire pier seems to dip down at one end once the collapse gets going. You can also see some sort of fence erected across it at the far side abutment. My guess is that they had been fortunate enough to notice movement in the piers before total failure.