PUP.Optional.SearchEngineHijack is Malwarebytes generic detection name for potentially unwanted programs that hijack the search queries on an affected browser.
Users will typically see search results not originating from the search provider they have chosen as their default search provider.
Search hijackers use different methods to get installed. In most cases they are browser extensions/add-ons that change the default search provider for the affected browser.
Usually they are installed by the users themselves under false pretences.
Malwarebytes protects users by blocking acces to the domains that spread and promote these extensions.
Malwarebytes can detect and remove PUP.Optional.SearchEngineHijack without further user interaction.