I'm also basing my answers on the links provided in the question. The interpretations are mine.
Erik V's reign was "snakebit" from the beginning. His father died when he was two years old. Any number of nobles tried to take advantage of his mother, and one of them, Duke Valdemar, succeeded in capturing the mother and Erik himself. He spent part of his childhood as a prisoner of the Duke.
When Erik grew older, many of his mother's enemies became his enemies. To his credit, he managed to make peace with the Church (the Pope, actually), as a young man. But others were hard to placate. The nobles who had spent over twenty years "running wild" wasn't about to take orders from a young man. Nothing he did, or would do, with the possible exception of abdicating, could please them. Because he was basically the "first among equals," the nobles forced him to grant them a "handfaesting" that preserved noble powers while limiting his.
The nobles were not content to stop there. They bribed his "best friend," Rane Jonson, to spy on him. Then the "friend" set him up for a bloody murder.
The handfaestning of Erik V was not like the Magna Charta. The former was unique to King Erik V, and each subsequent Danish king or queen signed a different document with his or her nobles until absolute monarchy was established in 1660. The latter was a permanent document that bound all kings of England, including Queen Elizabeth II, for all time.