The only explanation I can think of is the vent stack - which lets air into your drain piping - has a crack below the roofline and leaks when it rains.
Animals generally do not enter these pipes as the smell of sewer gasses are present, and they know enough not to try to inhabit this area, so its not meant as an animal deterrent.
I typically will recommend a licensed plumber evaluate the reason this exists so you can rest assured there are no cracks in this vital part of the plumbing system.
Ray Wilson is a Home Inspector, covering all of Long Island (Nassau and Suffolk), the 5 boroughs of NYC and now upstate NY. For any home inspection related questions he can be reached at Meticulous Home Inspection Corporation. He can be reached at 631-902-6761.