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Why were sepoys imprisoned at Meerut before the uprising began?

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Illustration of De Kantzow defending the treasury at Mainpuri during the Indian Rebellion of 1857, showing a British officer confronting armed sepoys inside a building as tensions rise during the uprising. — Indian Rebellion of 1857 Indian sepoys (soldiers) corner a British officer amid their attack on a treasury in Mainpuri, Uttar Pradesh.

The sepoys at Meerut were imprisoned because they refused to use the new Enfield rifle ammunition.1

The soldiers were required to bite off the ends of cartridges to load the rifles, but a widespread rumor held that the grease used on these cartridges was a mixture of pig and cow fat. Because this would have required oral contact with substances considered offensive or forbidden by their respective religions, many Muslim and Hindu troops rejected the ammunition. Those who refused to accept the cartridges were placed in irons by British authorities, an act that directly triggered the rescue attempt by their comrades and the subsequent uprising on May 10, 1857.21