Hijacking of websites is a practice that makes search engines believe that a specific website resides at another URL. It is a form of search engine spam and cloaking. The reason why this method is undertaken by spammers is to increase rankings in search engine result pages. Webpage Hijacking is an illegal spam tactic.
When spiders crawl websites and they discover two pages with the same content, the search engine will decide which is the main url while the other is not indexed. Spammers will use tactics to ensure that their page is the one that is chosen by the search engine.