To be sure, instead of one there were many and often conflicting causes to the rebellion of 1857. I've written on the topic before here, hopefully interesting for you (incl. discussion of terminology - "Sepoy mutiny" is a loaded term still today):
This one goes more into causes and previous resistance:
As an intro to those, from an earlier answer:
The rebellion took place 100 years after the beginning of the rule of the Easty India Company (EIC) in South Asia, and has to be understood as a continuation of a long tradition of local resistance to colonial rule (that continued up to independence). In this light, the rebellion was particular in its scope, but not in its goals and demands. There were many different kinds of and motivations for resistance: Among others religious insurrections, banditry, acts of revenge – but also peaceful opposition. Most of those insurrections were connected in some way to the very common economic exploitation and political marginalisation through the EIC, and thus to popular discontent.
It's also interesting to note that such discontent ran through all social classes. As a common ideology we can identify the central role of the community principle to the rural populace (including the caste system), and that of traditional land rights. Most (if not all) of these motivations for and forms of resistance played major roles during 1857. On the other hand, the lack of a coordinated, unified organisation meant that the British could count on parts of the local population, and was a major influence in the rebellion's failure.
Hope this helps!