Mostly food choices. Many factors: genetics plays some role. Parental modeling plays large role. Food available in community, tv ads, reduced physical activities, too much tv. The biggest factor is excess food and poor food choices. To help address the problem feed your child plenty of fruits and vegetables while eliminating all sugary beverages and try to cut back on junk foods. See choosemyplate.Gov.
A few more. Media + computer+ gaming use > 2 hours/ day is a huge contributor.Fast foods, sodas, junk food. Lack of daily outside activities. Childhood depression. Children who self-limit their food choices, either from permissive parenting or because they have an autistic spectrum disorders, eating/snacking anywhere other than at the table, food in large amounts used as a reward, unmonitored snacking.