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)
As an adult you joined a rebellion against the local ruler, if successful you are outlawed, if it failed you escape, with only friends and family knowing your involvement. (Work with your DM to detirmine the details)