It's only speculation that they did what to split from BP although I can see the advantages of more freedom.
All that we do know is that H&M are moving from under the KP household to the BP household which is the same as Charles is under Clarence House but Anne, Andrew and Edward are under BP. It's preparation for their future roles.
If it is true they wanted their own household I imagine it's because it's far easier to highlight the issues you want on your own than when you are caught up in the BP red tape. But as I say that's press speculation like loads of stories around H&M in media at the moment.