I agree with Kleinstein; the two series zener diodes clamp the output voltage and IC9 bootstraps zener diode D20 for minimum leakage.
I think the designer incorrectly assumed that D20 would have low leakage at low voltages without being a low leakage diode; bootstrapping it does lower the leakage but not to the level of a true low leakage diode. I would have used a low leakage diode, or two, in place of D20.