I've always thought of MM has this idea that the succession is like its own competition; thus creating all sorts of ideas to get her talking.
However, she must have realized that the Cambridges will always be in the front line. The priority, the center of attention and likewise.
When she figured out that once HRH Prince William will be Prince of Wales and the Cambridge children will take on the royal duties, Harry will be like Andrew, always at the back. MM will never wear the BRF jewels, she cannot have a big manor (cuz face it she wanted the whole Windsor Castle) and she'll never be invited at BP.
She must have idolized Wallis Simpson, who wanted out from Edward before the marriage.
Or maybe BP really wanted them out, so the both of jumped the gun, resigned, and placed two fingers on the Queen.