Like everyone else on this thread I do not personally know any of the individuals involved 
But, this is my take on a possible scenario?
MM is an engaging individual, with ambition and confidence, who PH fell in love with. He was very vocal about including her in his family and made reference to her fractured family very openly.
RF seemed to take to her (PC appeared genuinely proud to walk her down the aisle etc.) And given her lack of family ties across the pond they perhaps thought that the Sussex family would be securely RF focused.
But, perhaps MM really had no idea of what being in the RF would be like and thought she would be able to make changes, 'modernise' etc.
Perhaps PW had an idea of just how difficult it would be for MM because he'd seen what Kate had to go through and realised that MM was ill prepared. Perhaps he could tell that there could be a clash of approaches. Perhaps he was concerned that PH hadn't thought it all through and spoke to PH which resulted in the falling out and perhaps an even greater detirmination by them to "make a difference".
PH is besotted and can't understand that not everyone in his family will feel the same, so gets more and more frustrated.
That, together with his understandable hatred of the press and some genuinely appalling press coverage, has tipped him over the edge.
He feels rejected by his family, he feels that his wife has been disrespected, he feels undermined by his brother and family because they haven't embraced their vision.
But, instead of talking, working things through etc the Sussexes have decided to push their own agenda anyway. BUT, their agenda is dependent on being rooted in the RF 
So, they want to reject some of the constraints of the RF by not being "senior members" but retain a great many advantages by setting up their new venture with "royal" firmly in the title.
It feels to many as if they joined the firm, told the board of directors how they could do it better, and when tbe board didn't immediately acquiesce, they resigned but took the name of the company and the client list with them ...
It feels impulsive and immature, but also disrespectful.
It leaves many people wondering if there couldn't have been a better way of achieving their goal. Or, of being more "true" to their stated aim of giving Archie a more 'normal' upbringing.