“I’m sorry Luna, but it is time to put this to an end.”  
“How many times have I told you! Don’t call me-ARGH!”  
And then Nightmare Moon was the moon.
 
@Psychopomf I doubt Celestia would have used dark magic with the Elements. My gut tells me that the very nature of the Elements would prevent them from functioning at all if the user was actively channeling Dark Magic. More likely possibility is that not even a goddess can leverage the power of the elements alone. There must at least be two hearts for there to be harmony. No idea how she leveraged their power in that climatic fight. Maybe a part of Luna wasn’t full corrupted by the dark power that made her Nightmare Moon.

What if
 
The Elements rejected Celestia
 
After she channeled Dark Magic through them, while full of sadness and bitterness at her sister turning against her?