You were...idk something. You were transported, summoned, or sent...something just be something! Be one of the demon lords generals for all I care but you were disturbing the peace is something you did and you were spotted in the forest and you were placed a bounty for a hefty amount of eris and good enough for Kazuma to agree and hunt for you. So Kazuma, Megumin, Aqua, and Darkness went to the forest to hunt you so they become rich

Aqua: hehehehe! Can't wait to buy some more drinks

Kazuma: is that all you ever think about?

Darkness: Jeez...I hope the monster isn't to hard to defeat~

Megumin: don't worry ill just blast them away!