The Sepoy Rebellion of 1857 was the result of a number of aggressive policies imposed by the British upon their colonial Indian subjects in the years preceding the revolt. These included vast territorial expansion; divide and conquer military strategies; and practices which revealed a gross insensitivity towards religious norms that prevailed in the region, according to About.com.
British colonization in India was, from the start, primarily an economic venture. By the 1840s in particular, the British East India Company had to reverse financial setbacks, so huge new swaths of previously autonomous Indian territories were brought under British rule. As more territory came under control, more local Indian troops, referred to as Sepoys, were needed to police it, as the British regular army alone was far too small. As more Sepoys entered British service, they more often found themselves fighting against peoples from neighboring areas that they had previously not considered enemies. This generated considerable resentment against the British. To make matters worse, there were widespread allegations that the British used torture to secure cooperation from locals, a fear addressed by the Madras Native Association in 1856, one year before the rebellion.
The final straw was likely a religious one. Rifles of the day were typically used cartridges that contained both the projectile and the powder, and the soldier usually had to bite the tip off of the cartridge while loading the weapon. With the then new Enfield Rifle, the British issued cartridges sealed with beef tallow and pork fat. According to Emory University, most Sepoys were either Hindu or Muslim, and since each religion considers one of those food sources forbidden, many of the Sepoys became outraged. This apparently blatant and radical insensitivity toward Indian religious conviction further fueled the embers of rebellion, and some historians, notably J.A.B Palmer and John Kaye, insist that this was the immediate catalyst for the eventual bloodshed of 1857.