FAQ

Like the clothing and a lot of the surroundings, the dance wasn't historically accurate, but a funny approach to showcase the theme of the movie. Rather than being realistic and accurate, there was swinging, vogueing and some wild lifting in the dance routine, which wouldn't be appropriate during that time.

I believe there are two reasons. One could be because the Queen was jealous seeing lady Sarah dancing with a handsome man, and because she knew Sarah intentionally spiked that feeling in her, she slapped. Two could be because she got panic every time she heard the music (refering back when Abigail took her down the aisle and she screamed at the musicians to go off), so she was mad that lady Sarah had put her in that situation/ was unaware of her situation so she got furious. Then again, it could be a combination of both.