Albert's right, it must smell like the right place to off-load. Now that it's marked with their reek they'll never stop going there. You can try to clean it or restrict them from the wrestling mat area in your house. We had similar trouble when we got our dog, seems the previous owners of our house had a dog that fancied a spot in the living room, our dog assumed that was the proper dumping location and went there as well. I removed the carpet, laid down new flooring and the problem was solved.
These are giant wrestling mats salvaged from a high school remodel project. There is no putting them away. They are, however, frequently cleaned and sanitized with bleach.
