As a diver descends, they have to add air to their BCD (Buoyancy Control Device) to maintain neutral buoyancy (and release air when they ascend).
Without that compensation step, as a diver descends their buoyancy becomes increasingly negative.
Add a level of narcosis for descending below 40m, mix in some excitement/stress, and some equipment configuration issues (e.g. too much weight), and the task-loading can cause some people to forget/skip routine techniques such as adding air to maintain neutral buoyancy.
Without that compensation step, as a diver descends their buoyancy becomes increasingly negative.
Add a level of narcosis for descending below 40m, mix in some excitement/stress, and some equipment configuration issues (e.g. too much weight), and the task-loading can cause some people to forget/skip routine techniques such as adding air to maintain neutral buoyancy.