When user is using google without login it uses cookies to track our activities for the session and suggest us what we might want to search based on all the users activities. It in a sense assumes that what a lot of people would have searched up for or looked up for after the following article or video would be highly relevant to the current users search. The same goes for sorting the result, it assumes what web page a lot of users visited after a search was done would be the same a new user is looking for when they lookup for the same search result. The reason i like to call it back search is that it uses the previous data to suggest the future of the content one would want to see.