Purpose of this Zener diode (reverse protection)
I want to understand what is the purpose of the Zener diode D5202 and
at what conditions does the Zener diode come into action?

If this were your circuit: -

Then you could say with a certain amount of authority that the zener diode prevents the gate-source from being over-driven by an excessive voltage fed in to the right side of the 10 kohm resistor. It's quite common practice to do this.