Since Julius Caesar's invasion of Brittania in 55 BC had failed, no Roman Emperor had set foot on the British island. But when Claudius became Emperor in 41 AD, he thought it would be worth the try, and invaded Britannia again. He had more success: the eastern part of the island was set under Roman control in 43 AD. Many Celtic tribes voluntarily allied with the Romans in trade of their safekeeping. So did the Iceni, of which Prasutagus was chieftain (or king, as the leaders of tribes used to call him themselves), and Boudica was queen. Claudius had fortresses built and troops installed in several places in eastern Brittania, and a Roman governor was appointed to keep an eye on the Celts.
The second Roman governor, Publius Ostorius Scapula, took all weapons from the tribes that he didn't trust completely, like the Iceni, even if they had surrendered. This was a smart move, since he initiated a couple of things that would highly upset the Celts. For example, in 49 AD, he set up a colonia around Camulodunum (now Colchester): a town that would serve as a homestead for Roman veterans. Because the colonia grew fast and the Britons were driven off their land to make way for the veteran homes. Several were enslaved by the retired legionaries, others were executed.
In 54 AD (Ostorius had died and had been succeeded by a less provoking administrator), Emperor Claudius was poisoned and Nero followed him on the throne. Nero ordered to build a temple to his predecessor at Camulodunum. Now the tribal people were obliged to pay for a place of worship for a man who had took their lands! And on top of that, Rome demanded a repayment of money that had been loaned to chieftains that weren't even alive anymore.