The big culprit is that we metabolize O
2. The second culprit is that we limit O
2 at depth, because at too high of a partial pressure it becomes toxic. The third culprit, is that above 20FSW or so, our PPO
2 can't get above 1.2atm which we have it set at, so to prevent losing all of our O
2, we turn off our O
2 solenoid. But, we're still metabolizing O
2 and we're off-gassing N
2, so it's very easy for our O
2 levels to go below levels needed to sustain life if you don't pay attention. People prone to not pay attention are not good candidates for a rebreather.
There's a longer answer, but I'm not up to trying to 'splain it at this point.