On that note, there's also a technique, forgot the name, but it's basically anti-SEO where they intentionally push a competitor's pages onto a PBN with the intent of getting it detected (and penalized) by Google.
Kept happening for months. I'd disavow them, 10x as many would show up weeks later. Eventually I lost interest fighting and just stopped disavowing them.
There was never any penalty and from what I could tell the extra links actually helped our rankings! After about 18 months the links stopped. No clue what happened.
Unless your competitor had some serious dosh to mass spam your site on PBN's, they would most likely being using a tool like Xrumer or GSA where you can purchase a 100k backlink blast for as little as $5