Who said anything about bumping anything? Certainly the steel members on one side could fail and the structure would twist toward that side.
There is a lateral movement, and I assume, the first section to hit the water would be closest to the failure point.
If this is correct, then the original failure must have been located around the tower on the South bank. As viewed in the image (showing the largest view), that would the the top left side of the tower across the river.
On the right side of this image and also on the opposite side of the river, is the lock and dam. Now you got me curious, if this might not be connected in some way.