The Favourite (2018)
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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. Edit
A recurring theme in this movie is sacrifice. Abigail tells of how she sacrificed her youth and innocence for her father. Sarah sacrificed her pride and honor to get a job for her embarrassing, failure of a cousin. Queen Anne sacrificed her relationship with her best friend for a beautiful mistress. In the end Abigail sacrifices her pride and dignity for money, power, and prestige.
I also thought, at the end, Queen Anne was pushing down on Abigail's head, with enough pressure to deliberately hurt her - because Queen Anne had viewed to hurt the rabbit. I thought this meant that Queen Anne was not fooled by Abigail.
-- At that very moment they both realized that they were screwed. The Queen lost someone whom loved her dearly and honestly, someone who would help her to save England; while Abigail realized that she had the title that she wanted but she was nothing different from a maid, ready to be replaced by the next favorite. Could be even worse that she had sacrified her dignity to serve an incompetent Queen in a country that was about to be perished. Edit