Why did English barons rebel against King John before the Magna Carta was issued?

The rebellion of the English barons against King John was driven by a combination of financial exploitation, military failure, and the king's disregard for established feudal law and custom.12

Key factors contributing to the unrest included:

• Financial Oppression: Following the loss of Normandy in 1204, John relied solely on English resources, leading to increasingly severe tax demands. He frequently levied heavy scutage (payments in lieu of military service) and imposed massive, arbitrary reliefs on barons inheriting lands.324
• Military Failure: John’s attempts to regain lost French territories were costly and unsuccessful, culminating in a defeat at the Battle of Bouvines in 1214. His insistence on military service in Poitou was particularly unpopular, especially among northern barons.12
• Abuse of Power: Unlike his predecessors, John spent most of his time in England, making his rule feel more oppressive and personal. He was perceived as ignoring the rights of the nobility and the church, prompting the barons to demand a formal guarantee of their liberties, modeled after the coronation charter of Henry I.243