Why today?

Of all the times for President Trump to bring back the Elizabeth Warren "Pocahontas" theme, he did it today.
Why?
It's the first day of the battle over Warren's Consumer Financial Protection Bureau.
Does Trump have the authority to hire a new leader of this department, or does the power reside with the department?
Trump reminds everyone today that he has the power - to upend the news cycle with a single word.