The process is just prone to doing that. It's a combination of things - uneven application of the likely hand applied chemical coating, handling the plates by

The process is just prone to doing that. It's a combination of things - uneven application of the likely hand applied chemical coating, handling the plates by the edges while loading and processing, unevenness of the metal frame surrounding the plate in the camera, older lenses which had vignetting and lower sharpness at the edges, processing chemicals seeping into the edges of the coating, etc etc.