The mutiny broke out in the Bengal army because it was only in the military sphere that Indians were organized. The pretext for revolt was the introduction of the new Enfield rifle. To load it, the sepoys had to bite off the ends of lubricated cartridges.

Why did the mutiny start?

Outbreak. On 29 March 1857 at Barrackpore, Sepoy Mangal Pandey of the 34th Bengal Native Infantry attacked his officers. When his comrades were ordered to restrain him they refused, but they stopped short of joining him in open revolt. ... The Mutiny proper began at Meerut on 10 May 1857.

Why did the Indian mutiny begin?

It began (May 10, 1857) at Meerut as a mutiny among 35,000 Indian troops (sepoys) in the Bengal army. The immediate cause was the introduction of cartridges lubricated with the fat of cows and pigs, a practice offensive to Hindus and Muslims. A more general cause was resentment at Westernization.