The rockfall clearly took place in the slope above and to the left of the truck ahead of us. Given what I can see here, my suspicion is that weathering and erosion of the mud/siltstone undercut the thick sandstone layer visible to the left of the light tower. With support removed, that sandstone layer then spalled off, and down into the road. Again, this is just a drive-by guess, but it fits well with what one can see in the photo.
Photo unmodified. March 9, 2012. FlashEarth location (Fairly confident about this one, but not certain).