Most common would be a combination of overloading the lumbar spine, muscle imbalances, and movement compensations. Young athletes often are placed on training regimens that are far too advanced for their current level of stability and strength. For example, your average high school football player performs Olympic style lifts in the 9th grade without earning the right to by preparing their body to execute those high level exercises. You will also find them doing heavy weight squats, bench press, and the list goes on.
Instead, young athletes should be on strength and conditioning programs that create stability and enhance mobility by correcting improper movement patterns. A young athlete who follows this principle will be less likely to develop low back pain because they will be training properly. Ensuring that your child's school has a certified athletic trainer on staff to evaluate and manage injuries is key to prevention and also to ensuring that the complaint of low back pain is not something more serious.