One might not realize it, but many people who already know how to swim will take a breathe in while lowering themselves into water. When diving to do a cannonball the first thing many swimmers will do is take a deep breathe. Someone who doesn't swim might... not.
A child who doesn't know how to swim but is copying what they see others doing might be breathing as normal, have exhaled recently, and when they release the handles/ladder and enter the pool begin to sink. Not knowing how to swim and now head-under-water they are unable to get air into their lungs to have any amount of buoyancy.
A child who doesn't know how to swim but is copying what they see others doing might be breathing as normal, have exhaled recently, and when they release the handles/ladder and enter the pool begin to sink. Not knowing how to swim and now head-under-water they are unable to get air into their lungs to have any amount of buoyancy.