Public employee unions do not appear to work on the trades model. Seniority is everything.
Why was Derek Chauvin in charge of training other police officers? Chances are, the collective bargaining agreement requires selecting training officers based on seniority. It probably comes with better pay and promotion opportunities.
As a result, older police officers with a reputation for citizen complaints are put in charge of teaching the new police officers. 20 years from now, those new officers will generate their own complaints, because that is how they were taught.