Lack of love, lack of understanding, not enough support, someone said hormones, romantic or identity problems, embarrasment, lack of individual interaction/communication problems, wondering (or knowing) where they fit in at school -if they feel "less than" or feeling alienated or isolated ~just some ideas.
Lots of things, everyone has different triggers.
It could be caused by social interactions, school, friends, strangers. or pressures and stress caused by family, friends, school, and life in general. hormones are a possibility as well.
Many children get depressed for all sorts of reasons like peer pressure, family issues wheather it's parents divorcing or a parent choosing a sibling over the other, school, friends, lack of friends, bullying, there's a lot of reasons.