If you go to this part of their website, then look through the video clips...you get an idea of what might have happened. To ME, it looks like the tanker went off the bridge trying to avoid being involved with a "Can-hauler" and 4-wheeler that were tangled up in front of him.
http://www.heraldtribune.com/article...806060419/1661
Watch the news conference clip, the news helicopter clip...and listen to a couple of the 911 calls.