What were the immediate (or short-term) causes of the Sepoy rebellion?
The immediate cause of the Sepoy rebellion was the rumor that the cartridges for the new Enfield rifle were lubricated with fat from pigs and cows.
A part of the cartridges for the new rifles had to be torn off by biting them with the teeth.
Here is an image of the Enfield rifle with its gunpowder cartridges.
But the Hindu soldiers were prohibited by their religion from oral contact with cows, which they considered holy.
The Muslim soldiers were prohibited from oral contact with pork, which they considered unclean,
The soldiers who refused to use the cartridges were arrested, which compounded the anger and disorder, and the mutiny spread.
Here is an Encyclopedia Britannica article describing the background that led up to the wide-spread but unsuccessful rebellion. This article calls the cartridge problem a "pretext."