A short summary of the known mechanisms of oxygen induced hypercapnia:
V/Q Mismatch
PaO2 rises and reverses hypoxic vasoconstriction, resulting in increased dead space ventilation and rising PaCO2.
Ventilatory Drive
Reversing hypoxaemia causes the ventilatory drive to reduce and ventilation to decrease, which can raise PCO2.
This is the case for all patients and seems to only play a small role in the hypercapnia.
Haldane Effect
This is a property of haemoglobin.
An increased FiO2 causes the buffering capacity of haemoglobin for CO2 to reduce.
Absorption Atelectasis
Thought to occur when O2 is absorbed from alveoli with high PaO2 beyond obstructed airways, causing atelectasis when that gas is absorbed and leaves the alveoli.
Rebreathing from face masks
at low flow rates, causing the exhaled CO2 to be inhaled.
Higher Density of O2
Oxygen has slightly higher density and viscosity than air, likely increasing the work of breathing and contributing to hypercapnia.