What was the main cause of Sepoy Mutiny?

The main cause of the Sepoy Mutiny was the introduction of new rifle cartridges that were thought to be lubricated with grease containing cows and pig fat. Since the new cartridge needed to be bitten before beng loaded into the rifle, this hurt religious sentiments of both Hindu and Muslim soldiers.

The rebelling sepoys were shackled and imprisoned, but their outraged comrades shot their British officers and marched on Delhi, thus beginning the 1857 rebellion.

For further reading check the following articles:

• Revolt of 1857: Economic Factors
• Revolt of 1857: Immediate Factor
• Revolt of 1857: Observations by Historians