What British action caused the Sepoy's refusal of the cartridges to escalate?

The Mutiny began in earnest at Meerut on 10 May 1857 when 85 members of the 3rd (Bengal) Light Cavalry who had been imprisoned for refusing the cartridges were rescued by Indian comrades. The following day Delhi fell to the mutineers. News of these events spread rapidly, leading to further mutinies elsewhere. So the British action was imprisoning the Sepoy's for it.