Drowning in shallow water is commonly caused by breath-holding activity. The victim (abled swimmers/freedivers) will hold breath as long as they can resulting in a blackout.
Usually, for swimmers, the training involves 25m / 50m swim without taking a breath.
For freedivers, it's a static breath-holding (without any movement) as well as a dynamic movement such as finning underwater for 50m or more.
The main reason why holding breath is common amongst elite/competitive athletes is to improve Carbon Dioxide (Co2) tolerance in the bloodstream. So when they compete with full gas, the performance will be enhanced, and for freediving, they can stay underneath without oxygen for a longer time.
Warning for parents & competitive athletes:
1) Do not let your children performing breath-holding training without adults supervision.
2) For competitive athletes, do not perform this after training session finished / without coach's presence.
3) For freedivers, do not do this breath-holding training without qualified/trained buddy or instructor.
Thanks to MySwim coach, Rose Lizzy for highlighting the issue. It is a question from a parent to us from this video