If you don’t believe that a rotten, nasty, dishonourable thing like negative SEO exists, it’s time to think again.
There’s no way I’m going to give this lot a link, even a nofollow link, but take a look at Unranker.com, using Google Translate if you don’t speak the lingo.
They’re offering a thoroughly black hat negative-seo (AKA hostile seo) ‘unranking’ service where they’ll attempt to de-rank target sites, in other words sites that compete with their customers’ businesses in the search engine ranking pages.
The people offering the service make it clear that not every site can be adversely affected. But if they think it can be done, they appear unpleasantly confident that negative results, for example a minus 50 Google penalty, can be achieved within four weeks. If they don’t manage to trash your competitors’ search positions, you get your money back.
Nasty, nasty, nasty.
If your website suddenly drops in the SERPs for no apparent reason, negative SEO might be the culprit.