Hmm...lots of reasons.
If they're a kid, it might be because they were being abused at home, or they were seeing their family fight, seen a death of a loved one or friend, seen a school shootinng, ect.
or
If they're an adult, they might want to get away from their career and start a new one as a entirely different person(this mostly goes for singers and dancers, but buisness-people as well). ect.