An important part of Google's algorithm is duplicate content. If multiple pages on the web have the same content, only the one that Google believes is the original/authority remains. This is a technique to fool Google into thinking that the wrong one is the original, therefore deindexing the real deal.
Page hijacking is creating a copy of a popular website that shows contents that is similar to the original. This technique can be used by the spammers to achieve higher rankings for some of the keywords.
The activity of stealing content from a Web site and copying it into another Web site in order with some of the original sites traffic to the copied Web pages.Then copied site will appear in the search result rankings along with the original site.