Well, it all started with the death of King Prasutagus, who was the king of the Iceni tribe in Norfolk. When he died he left his kingdom jointly to his two daughters and Nero, the current Roman emperor in an attempt to keep both Rome, and his daugthers happy, allowing the Iceni to still retain some independence, and not be held directly under Rome. However, his request was completely ignored, and the kingdom was annexed as if in an invasion. The debt that Prasutagus owed the Romans may also have had an impact in their choice to disregard his will, and thought the entire lands payment. Perhaps the flogging and rapes (not only of Boudica and her daughters, but the general populace) and looting may have been to show that the romans were indeed in control, and to force the Iceni into submission. Which evidently did not work.

Sources: A whole host of webpages and books, but the most notable (becuase of when they were written) are the Annals by Tacitus, specifically section 14.31 and Roman History by Cassius.