Because bad and unnatural links can get a website penalized by Google, some shady companies will purposefully build and point bad links to another website with the specific intent of getting them penalized. It might be a competitor of yours, or a black hat SEO agency they hired to give them an edge. Google has claimed these attacks don’t work, but our own experience has shown otherwise. We still recommend our clients attempt to clean up and disavow these as they can cause ranking issues, both now and in the future when Google launches major algorithm updates.