I highly doubt that these “shake downs” are done at random.
The Japanese are consummate planners, they leave nothing to chance and no bureaucrat is allowed / able to improvise or make decisions on hunches.
Therefore I believe they have an action plan with which they can reach an annual financial target, as in money collected from gaijins who‘s been here long enough and might have acquired assets.
This flowchart might be one approach:

I choose 6 years as the residency threshold because we file taxes for the previous year, so filing taxes after being here for 5 years would mean the income for year 4.
The age limit of 30 is arbitrary however if a foreigner’s been living in Japan for 5 years and he/she is under 30 that means he/she came over fresh out of university which would mean he/she had earned very little money and probably doesn't have a high paying job in Japan either.
The income limit of 5 mil is again arbitrary, however given the high cost of living in Japan and young people’s chronic inability to save anything substantial while living on their own would suggest that earnings under that amount would not allow a person to save significant amounts. Your mileage, of course, may vary.
The "trigger event" of course means a large cash remittance in / out of Japan, large cash deposit, etc.
Amend as you wish.