R&B singer Chris Brown's recent dumping of girlfriend Karrueche Tran stems from an ultimatum Rihanna gave him, due to her ego and wounded pride. Rihanna has been embarrassed and jealous of Brown publicly making a big deal of Tran and wanted her out of the picture. Rihanna sees herself as a big star and Tran as a nobody she should not play second best to (yes, a legend in her own mind, rather than in her own time, from a career based on fraud and copyright infringement). 

Rihanna let him know, if he wants her to continue privately seeing him and showing him sympathy in public, which they deem will help his damaged image and career, he would have to get rid of Tran, whom he fell in love with. Chris Brown's number one priority is not Rihanna or Tran, it's his career. Brown's number one priority is himself. So he dumped Tran.