I did a little looking with Google earth and their ground views from the road that passes under the bridge. That series of photos, from the road, as well as the airphoto show no fill on site. However, there is fill there now and that probably is involved. See this video and the large pile that they are now starting to remove. My money now is on that fill affecting adjacent ground.
I'd also agree that the prominent fill adjacent to the column(s) is directly realted to the tilt. I'm just struggling to reckon why the column tilted toward the fill rather than away from the fill.
Possibly the embankment fill caused the weak soil below to push toward and bend the deeper portion of the long foundation piles away from the embankment, causing the piers to tilt toward the embankment.
. . . and also, the pile cap could have moved away from the fill, thus making the column appear tilted toward the pile. Sort of the hinge point. So, the bridge deck didn't move at all.
I'm having trouble finding anything to scale to judge by and also seeing the tilt.
However, could the pile of soil have caused a bearing capacity failure? If so, it could have caused the soil below the pier to move away from and upward; thereby causing the top of the pier to tilt toward the pile of soil.
Envision a slip circle movement with the center of rotation somewhere between the bridge and the fill pile. There may not necessarily be a heave of ground where you have very soft stuff, capable of compressing as it is pushed sideways. And not all slip failures are a perfect circle thing. An explanation as to where those ground surface cracks were first found might help.
