Shunt regultor, 431, is failure during testing Electrostatic discharge(ESD). The testing conditon is 10 times 8KV in contact and 10 times 15KV in air.
I think clearance is short between Vout and 431 on PCB. However, it is not enough to explain and report.
So, let me know why it is happen and what I should do not for happening again in the future.
You can see the part of circuit and PCB below.