Why was the Duchess put in prison?

The Duchess was in prison under sentence of execution because she boxed the Queen's ears.

‘Hush! Hush!’ said the Rabbit in a low, hurried tone. He looked anxiously over his shoulder as he spoke, and then raised himself upon tiptoe, put his mouth close to her ear, and whispered ‘She’s under sentence of execution.’

‘What for?’ said Alice.

‘Did you say “What a pity!”?’ the Rabbit asked.

‘No, I didn’t,’ said Alice: ‘I don’t think it’s at all a pity. I said “What for?”’

‘She boxed the Queen’s ears—’ the Rabbit began. Alice gave a little scream of laughter. ‘Oh, hush!’ the Rabbit whispered in a frightened tone. ‘The Queen will hear you! You see, she came rather late, and the Queen said—’