The only thing I can think of is to prevent damage to the 7660 when the power is turned off. It is possible, although I wouldn't think likely, that the positive supply could couple throught the FETs to the output of the 7660. Other than that I can't see it ever conducts.
The question presumes that the diode actually serves a purpose. You should also consider that it might be unnecessary.
ICL7660 has already a substrate diode which becomes forward biased when positive currents are fed to the output node. A schottky diode can be meaningful in cases when the current could cause latch-up. The 10k series resistor will prevent this safely.
Most likely, the author copied the diode from a different application without understanding it's purpose.