Boromir's guess the famous game #223 [Boudica is the famous person, Carrot wins!]

It's difficult to stop a thing like this once it gets moving. Even if Boudica had been inclined to do so, many among her people didn't want to stop until complete victory was achieved, and she might not have been able to hold them back. What the rebels wanted, not just Boudica but the whole people, what they wanted was to throw Roman rule off entirely, not merely to reform it. Considering that the straw which broke the camel's back was the Romans violating terms, the Celts had no reason to trust the Romans to honor any agreement they might make with them. It was preferable in their minds to expel the Romans from the island entirely. Considering the Romans had left Britain after Julius Caesar invaded it a century before, this wouldn't have seemed out of the realm of possibility. This was also part of the motivation for exterminating the Roman citizen colonies, which were an anchor for Roman rule on the island (the entire reason they were established). The other reason being that the colonies were centers of systematic oppression and abuse against the native population.

From Tacitus:
"Prasutagus, king of the Iceni, famed for his long prosperity, had made the emperor his heir along with his two daughters, under the impression that this token of submission would put his kingdom and his house out of the reach of wrong. But the reverse was the result, so much so that his kingdom was plundered by centurions, his house by slaves, as if they were the spoils of war. First, his wife Boudicea was scourged, and his daughters outraged. All the chief men of the Iceni, as if Rome had received the whole country as a gift, were stript of their ancestral possessions, and the king's relatives were made slaves. Roused by these insults and the dread of worse, reduced as they now were into the condition of a province, they flew to arms and stirred to revolt the Trinobantes and others who, not yet cowed by slavery, had agreed in secret conspiracy to reclaim their freedom. It was against the veterans that their hatred was most intense. For these new settlers in the colony of Camulodunum drove people out of their houses, ejected them from their farms, called them captives and slaves, and the lawlessness of the veterans was encouraged by the soldiers, who lived a similar life and hoped for similar licence. A temple also erected to the Divine Claudius was ever before their eyes, a citadel, as it seemed, of perpetual tyranny. Men chosen as priests had to squander their whole fortunes under the pretence of a religious ceremonial. It appeared too no difficult matter to destroy the colony, undefended as it was by fortifications, a precaution neglected by our generals, while they thought more of what was agreeable than of what was expedient."

So it wasn't just Boudica; it was everybody that was suffering at the hands of the Romans on the island, who systematically robbed and abused the people of all classes with impunity. This is what provoked the rebellion.

Not to go on, but just one more point. In reference to the earlier argument about Rome being at the height of its power and therefore rebellion against it to be futile. It didn't occur to me at that moment but Boudica would certainly have been aware of the fact that about 50 years before her revolt the Romans had been successfully defeated by the Germanic tribes in the Battle of the Teutoburg Forest, in which Arminius completely destroyed three legions, putting an end forever to Roman plans to conquer Germany beyond the Rhine. Indeed Tacitus claims that Boudica took inspiration from the Germans. He provides a speech he attributes to Boudica, although it should be noted that this would have been dramatic license on his part as there's no way he could have known what exactly Boudica said (and writing battle speeches for foreign leaders was a common thing for Roman historians, in order to convey the leaders' motivations), but in the speech he has Boudica saying that Germany shook off the yoke of Roman rule and that they were divided from Rome by only a river, not an ocean as the Britons had. Again these wouldn't have been her exact words, but Boudica would definitely have known about Teutoburg, which was infamous in the Roman world during her lifetime, and would have taken inspiration from it. Teutoburg showed that the Romans could be defeated and driven off for good.