Final answer:
Sir Thomas Malory was imprisoned due to his role in the Lancastrian rebellion against the Yorkist King Edward IV during the Wars of the Roses.
Step-by-step explanation:
Sir Thomas Malory was imprisoned in 1468 for his involvement in the Lancastrian revolt against the Yorkist king, Edward IV. This period in English history is characterized by a series of dynastic wars, known as the Wars of the Roses, which saw the houses of Lancaster and York in conflict for control of the English throne. The War of the Roses finally ended when Henry Tudor, a Lancastrian, defeated his Yorkist rival in 1485 and established the Tudor dynasty, thus bringing major political changes to England.