Not sure if anyone posted this already, but this all started with satire.
About 10 years ago, there was an article in the Economist that imagined the US government if it was run like the French, ie, presidents having obvious mistresses etc. The example was Obama and Aniston. This was then picked up by a gossip merchant on a subscriber only channel... and they made it a "thing".
Of course, life can end up imitating art, but it's been mentioned a couple of times in the past 10 years but, wildly, only now is it getting traction.
Michelle is a huge public figure in her own right, and has a juggernaut of a tour/ speaking/engagements packed calendar. She's also clear on no longer wanting to be the "plus one", so I don't see how not attending official engagements would be an f you to Obama. If she didn't go to his birthday, on separate holidays, maybe, but can't stand to be with him at a gathering of 100s? Makes no sense purely on that basis.
As to William, whoever said he's too nice, well bless your heart, but sometimes that's not enough.