The Sepoy Rebellion of 1857 occurred for a few different reasons. One reason was that the Sepoy soldiers, most of whom were either Muslim or Hindu, were forced to use paper wrapped rifle cartridges dripping in beef and pork fat. Having to bite the paper wrappers off these cartridges violated the religious practices of these soldiers, which angered many of them and contributed to the eventually rebellion.
Another factor was the recent reorganization of the Indian military, which angered many Indian elites and officers. The Indian part of the army outnumbered the British part 5-1, so when the rebellion broke out they were able to make early gains thanks to sheer volume of solider.
The catalyst was the murder of a solider by the name of Mangal Pandey, who stuck one of his superiors out of anger over recent changes. He was eventually killed, which infuriated the rest of his unit and eventually led to a rebellion which nearly toppled British rule in India.