Breakthrough:
You can see it quite clearly if you buy the kind of scrubbber with a dye, but obviously the scrubber at the bottom gets used up first and above that there is a cone of used up scrubber material. With time the cone gets bigger and gets closer to the top until the tip of the cone finally breaks through the top of the cannister. Like electricity taking the easiest route in lightning, the CO2 takes the easiest route through the scrubber cannister: up the cone and through its tip. There is now a route through the cannister through which CO2 can bypass the scrubber material. The diver's inhaled FCO2 starts to rise as the breakthrough hole gets larger and larger.

A radial scrubber is effectively a very short fat scrubber cannister. The incoming gas works on a much larger surface area of scrubber material, which means the work of breathing is much lower, but when it breaks through it breaks through in style. The diver's FCO2 then rises very sharply. The narcotic effect and panic response take out the diver's reasoned thinking, increasing his response time, and a number of other effects effectively pull out his power cord, reducing his ability to respond.