Out of all the people who've tried suing over or aggrieving a reassignment, only 1 person has won. That person won because he blew the whistle before it happened and he was widely known as a better detective than most others in his unit. It wasn't easy to prove he was transferred for whistleblower retaliation. Without a smoking gun like that, the Chief can otherwise transfer you in the best interests of the department and doesn't have to provide an explanation about what that is.