The mutiny was, literally, triggered by a gun. Sepoys throughout India were issued with a new rifle, the Pattern of 1853 Enfield Percussion cap rifled musket - a more powerful and accurate weapon than the rifle they had been using for the last several decades. To load the new Enfield, just like the previous muskets they were issued, soldiers had to bite the cartridge open and pour the gunpowder it contained into the rifle's muzzle, then stuff the cartridge case, which was typically paper coated with some kind of grease to make it waterproof, into the musket as wadding, before loading it with a ball.
A rumor spread that the cartridges issued with this rifle were greased with lard (pork fat) or tallow (beef fat) - this was offensive to Hindu and Muslim soldiers alike, who were forbidden by their religions to eat beef or pork.
