The mechanism that was thought to be responsible for CO2 retention and has been taught to many doctors in medical school is that of the Loss of Hypoxic Drive
The theory was that chronically high CO2 would cause the body to rely less on the central chemoreceptors, driven by CO2, and more on the peripheral chemoreceptors, driven by O2.
This would have caused CO2 to rise when supplementing oxygen since the stimulus to breath would be reduced, i.e. "no longer driven by hypoxia".
However, this was based on a misquote of Dr Campbell in 1967, who himself doubted this would be the main cause of hypercapnia, and more recent studies have shown that the mechanisms for CO2 retention are much more complex.