Apparently, after Shichibukai were abolished, Mihawk decided to finally form a Crew, Crocodile thought it would be great chance to recruit other Shichibukai to create Yonko Level Force he always wanted, and Bakkin thought the same, Hancock was able to defend her Island thanks to help from one of them (They wanted to recruit her first since she is easiest to find)
However Buggy somehow outsmarted them and made the world believe that he recruited all of them under him, so they called him a New Emperor