Erosion of bedrock depends on how competent it is, and zones of weakness can erode much faster than the bulk rock. Weathering and weaknesses in the bedrock below the main Oroville spillway shaped the pattern of erosion once the cement of the spillway failed. Erosion at the base of the auxiliary spillway accelerated once a zone of weakness eroded into a gully.
Here is a great article by UCD's Professor Emeritus Eldridge Moores discussing how weathering of the bedrock influenced erosion on the Oroville spillways: How 'Incompetent' Rock Led to the Oroville Dam Crisis
Here is professional publication about how variable bedrock erosion rates can be, depending on zones of weakness: Exceptional river gorge formation from unexceptional floods. This type of erosion occurred below the auxiliary spillway in the "danger area" marked in the image below.