What are the causes of a teen to be overweight?

Over eating: Unless there are specific genetic problems, such a prader-willi syndrome, the main reason behind being overweight is ingesting more calories that you need, in other words over eating.

Multiple factors: Generally, a young person who is overweight is not exercising enough (a minimum is 150 minutes per week in youth) and making poor food selections. A part of the problem typically is too much caloric intake. There is a correlation of "screen time" with obesity. More screen time -> more weight. Finally, in some stress or depression can contribute. Start with more exercise and smart eating.

Same as in adult.: Teenagers these days are more sedentary than they used to be - more homework, more video games and texting, and less time running around outside. Just like an adult with a desk job who has trouble losing weight. Bad food choices that teenagers generally make only compounds the problem.