A vicious animal attacks you on a moonlit night. Friends drive the beast off, but you discover that you've been cursed with lycanthropy, to become a similar beast under the full moon. (work with your DM to determine details)
Poverty You or your family fell on hard times and were unable to pay taxes, resulting in either your character or a family member spending time in prison. Work with your DM to detirmine the details)
In adult life you gain a rival who seeks to undo your character, they will attempt to hinder your efforts until they get what they want. (work with your DM to determine details)