This effect can happen when using medium format and larger negative carriers as they use glass to keep the negative flat. Most carriers today use a frosted piece of glass above the negative known as anti Newton glass.
The rings occur in the areas where two clear smooth flat pieces of glass lie next to each other but do not quite touch. The interference happens because the light rays are refracted, this produces irregular shaped dark rings in the projected image. This will happen more readily if any of the surfaces are wet. You can get round this by using a paper mask with the negative in the carrier. This will block out the light in the places where there is a gap between the glass surfaces. You will need to check the projected image to see if they are still visible.