My take on it is this:
Charlotte clearly has had her moments (cue picture of her sitting down on the tarmac in a sulk, the line “You’re not coming” to the photographers at Louis’s christening).
I suspect that Charlotte was being less than cooperative in the way that two year olds are - Terrible Twos and all that - the nanny was at home with George and Louis, Kate was exhausted, hormonal didn’t need any more frustration.
And then MM snaps and says something along the lines of (pure speculation) Charlotte stop messing about FGS Kate can’t you control your own child?
Boom.