Not a mystery. This is directly correlated to the CPAP's primary goal which is to ensure a steady flow of air in and out of your lungs. Without the CPAP, your heart is reacting to variations in O2 (inflow) and CO2 (outflow), speeding up and slowing down accordingly, including experiencing stress during periods where the airway is completely obstructed.

I think that's backwards, higher HRV is better?