Earlier quoted context omitted.
This is only because you're not able to recognize your urge to breathe for what it is...a buildup of CO2 rather than a need for O2. With practice in breathing exercises, you learn to recognize the start of contractions (in the diaphragm) as a stage in the breath hold that's typically around 1/3 of your maximum. With more practice, you can start to recognize more subtle symptoms of low O2...tingling in the extremities…
> And running out of O2 underwater leads to black outs, especially in shallow water as water pressure drops fastest when surfacing those last 10m-20m. Because people swim faster when they're close to the top?
But that's not the issue here. It's the pressure difference when ascending.
> [1] Ascent-induced hypoxia is caused by a drop in ppO2 as ambient pressure is reduced on ascent. The ppO2 at depth, under pressure, may be sufficient to maintain consciousness but only at that depth and not at the reduced pressures in the shallower waters above or at the surface