The issue that led King John to sign the Magna Carta was that when King John was ruling, there was a war going on, and he kept on asking for more soldiers and more money/taxes from the Nobles/Lords and citizens. Of course, the Lords and Nobles didn't like that the King was doing this, so they forced him to sign the Magna Carta, giving rights and limitations to the people.
Before the signing of the Magna Carta, the King could do as he pleased.
