I did SEO at the now defunct web-house Crexendo. Some of our clients were big businesses that I can't name. BTW, my non-compete clause is over so I can SEO again
So, it is called "reverse-SEO" and it works like this:
Sometimes sites will pay money for other sites to be buried in the search results (usually competitors or bad reviews). Occasionally, tactics such as unrelated posts will be deployed which when crawled by the bots can negatively affect your site (if not relevant).
Not through us, but Toyota paid $500 million dollars (according to an SEO expert on NPR) to have all sites that posted negative reviews to be buried past page 1 of Google search results.
That is my best guess, although it is usually used for one's own site as a sort of 'damage control' rather than sabotage of competitors.