The methodology I believe follows the CFA MPS methodology closely.
This uses a modified Angoff method which basically surveys a large panel of diverse experts on each individual question on what probability that a just-good-enough candidate would get this question right.
This process is repeated several times at different levels, and psychometricians oversee the process. This input is used as the main reference for setting the MPS.
The objective is to set a consistent competency level of passing candidates across years.