It started with a fire in Calcutta, circa 1857, here is one summary. Excerpt:
The insurrection was sparked by the introduction of cartridges rumored to have been greased with pig or cow fat, which was offensive to the religious beliefs of Muslim and Hindu sepoys (soldiers). In a wider sense, the insurrection was a reaction by the indigenous population to rapid changes in the social order engineered by the British over the preceding century and an abortive attempt by the Muslims to resurrect a dying political order.
Here is how the Arabs get all those Danish flags, thanks to several readers for the pointer. The current price is $11 a piece.