How did the Indian mutiny in 1857 begin?

The Indian Mutiny was a traditionalist revolt against a wave of administrative, technological, and social reforms and became focused on the perceived British attack on the Hindu Caste System, driven by the fact that the Bengal army sepoys were largely high-caste Brahmins.

The cause was the General Service Enlistment Act of 1856 which required troops to serve overseas, itself a threat to caste, as well as involving voyages by sea where the risk of contamination in confined quarters from those of a lower caste by shared messing and sanitary facilities was ever present. The famous greased (in fact waxed) cartridge introduced the same year proved to be the last straw. It was rumoured that they were lubricated with grease from cattle (holy to Hindus) or pigs (unclean to Muslims). On 29 March sepoy Mangal Pandy of the 34th Native Infantry ran amok on the parade ground at Barrackpore and not only he but also a native officer who had failed to stop him were hanged.

On Sunday 9 May 1857, after some troopers had previously refused the new load and had been drummed out, the 3rd Light Cavalry and 11th and 20th Native Infantry at Meerut (Mirath), near Delhi, opened fire on their officers at church parade. In the absence of prompt action by the local British commander, they massacred the British families and then marched on Delhi, where the garrison also mutinied and massacred the Europeans.