"Don't say that to BOTH kids--like Grandma!"

For the last year we've been trying to figure out what it means when AKA suddenly explodes and yells this phrase, and why it often precedes a meltdown. She used to just say it to my mom, at least once a visit, randomly. But now it's being used on anybody and everybody, and until now the reason for it was a mystery. Yet lately a pattern is being revealed, that it has something to do with more than one person talking at once. So when we hear "Don't say that to BOTH kids--like Grandma!" (the last part is always emphasized) we know AKA's mood has gone sour and it's time to back away slowly, flee the scene, etc.