My Little Pony: Friendship is Magic - Image #778,778

YEAH THREESOME!!!

Well, there was the whole "Mind Control" thing going on.
It wouldn't be too much of a stretch to imagine that Shining Armor wasn't the only one getting zapped with magical hypnosis. Chrysalis-as-Cadence was in Canterlot for a while, she could have been using a little bit of it on Celestia during that time.

I know, it's a shaky excuse, but better than nothing.

What's dumber is the fact that Chrysalis was able to beat Celestia in single combat. That's complete bullshit if you ask me.