There are at least two ways to answer this question.
First, we can look at the ultimate causes of Boudicca’s rebellion. The Romans had conquered Britain in 43 BC. By Boudicca’s time, about 100 years later, not all Britons were completely comfortable with the idea of being ruled by the Romans. Some British tribes, like the Iceni (of whom Boudicca was queen) valued their independence. This desire to be independent was surely an important reason why Boudicca rebelled.
However, there is also a more immediate cause of the rebellion. That was the fact that Boudicca’s husband, Prasutagus died after having been the king of the Iceni. Prasutagus had been ruling the Iceni as sort of a client ruler for the Romans. The Romans liked to install client rulers who could help them control a subject population. They would eventually try to take direct control over the client state. That is what happened why Prasutagus died. His will left the kingdom jointly to his daughters and to Rome. But the Roman officials wanted to keep the kingdom all for themselves and overrode the will.
When they came to enforce this decision, Boudicca was stripped and beaten and her daughters were raped. Because of these outrages, Boudicca started the rebellion.
Thus, we can say that Boudicca instigated the rebellion partly because of a general desire for independence and partly because of her desire for revenge for the way she and her daughters had been treated.