The hive are not entirely leaderless. Oryx may have been the god king of the hive but there would have been other influences such as savathun and xivu arath. The chain of command means that after oryx's death one of his sisters would take his place which in this case is savathun commanding them possibly from her ascendant realm because during the savathuns song strike the faction of hive loyal to savathun were trying trying ti summon her to the material plane most likely to secure titan and to claim oryx's two armies