It's been five years since the Vault was opened on the planet of Pandora. The first Vault Hunters were lied to. It contained no treasure, just an alien horror locked away in hibernation. After slaying it, the opening had another effect. Eridium. This precious mineral began to literally erupt from the ground, and even corrupted a vast area of land. Handsome Jack, the CEO of the Hyperion Corporation, has taken all the credit for the opening of the Vault, Eridium, and the E-Tech technology he was able to engineer using it. It made him an overnight trillionaire.
Using an army of Hyperion loader robots and shock troops, took on the task of 'liberating' Pandora from its outlaw population. That is, killing anyone or anything that got in the way of a new goal.
He learned of a second Vault, that contained something referred to as 'The Warrior'. In order to stop others from awakening the Warrior before he could, Jack sent an off-world communique, hoping to attract treasure hunters to Pandora, where he would have them killed before they reached their goal. Little did he know that some treasure hunters don't die so easily.