When the paramount chief Powhatan died in 1618, leadership passed to his brother, Opitchapam. Although second in command, another brother, Opechancanough, soon became the real leader and planned a co-ordinated assault on the encroaching English settlements.
On the day of the attacks, warriors wore no face paint nor carried weapons to give the illusion that it was an ordinary one for work or trade. At the agreed upon time, they turned on the colonists and the bloodshed began, oftentimes using the colonists' own weapons or tools.
By the end of the day, over 300 colonists, equaling a third of the population, were dead. On both sides of the James River, the death toll included women and children. Until the annihilation of the Paspahegh in 1610, such a tactic had been uncharacteristic for Virginia Indians as it had been against the law of nations. The previous hostilities with the colonists had forever changed the nature of their warfare.
Legend states that a Native Christian boy by the name of Chanco saved Jamestown. The story goes that he warned his employer, Richard Pace, of the upcoming assault. Pace lived across the river from Jamestown, so he rowed to the colony to alert them before the impending disaster. As it turns out, there's no historical or archaeological evidence that Jamestown was ever threatened.
To explain the possible reason further, I consulted with a tribal historian and was informed a treaty had been in place since 1614 giving Jamestown to the colonists. During the time of peace before Powhatan's death, the colonists encroached further into Indian territory. Opechancanough's attacks were co-ordinated to enforce the treaty. Essentially it was his way of saying, the land belonged to the Native people, and the colonists needed to live by the rules they had agreed to.
The attacks were carried out in typical Indian fashion of striking as quickly and as hard as possible, then retreat. At that time, the Powhatan had the larger numbers and could have carried out even greater damage had they wished, which makes the treaty explanation very plausible.
In the end, the colonists responded by increasing their attacks on the indigenous population and seizing more of their land. Although Opechancanough is often portrayed as a villain in modern history books, he remains a hero to present-day Virginia Indians as a defender of their homelands.