This is an interesting question and something military historians have been trying to answer since more than a 100 years.
Enfield rifles were used by the British Army in the mid 19th Century and most of them used grease made of animal lard.
The East Indian company had conscripted an Army of locals within India. These were made up of mostly Hindu and some Muslims men.
Rumors began to circulate that the grease of made of cow or pig lard. This was highly offensive to Hindus and Muslims alike, since Hindus considered the Cow to be their god and Muslims considered pig to be unclean.
To how large of an extent did this play in turning into the 1857 Rebellion is arguable, however, it did inflame tensions between the occupied and the occupiers.
