You survive a deadly encounter with a non-humanoid monster. Because of the encounter, you gained insight into the creatures weaknesses. (work with your DM to determine details)
Divorce: During your adolescence your parents split up and you were forced to chose which parent to live with (Work with your DM to detirmine the details)
Approaching adulthood you experience wanderlust and decide to travel, roll 1d6 to detirmine years spent travelling and think about where your character might have gone. (work with your DM to determine details)