Someday, we should probably have an intervention with J.K. Rowling over how much time she spends on Twitter. There are other things she could be doing than engaging with the Harry Potter hordes. Today isn't that day.
Today is the day in which she corrects our mispronunciation of He Who Must Not Be Named.
Since 'mort' (with a silent 'T') is the French word for 'death', that's probably why it's supposed to be pronounced that. But who knows. I'm sure Rowling will go into greater Twitter detail in the coming weeks and months.
Everyone probably started calling him 'You Know Who' because no one could pronounce his damn name.