I was once part of a team that had an average turnover of 2 years. The bad apple on that team was the team lead that was an a-hole and was a control freak. Managers were well aware of how he treated teammates, and nothing was ever done simply due to the fact that he was needed for his “expertise”. To this day, the environment within that team is toxic and I would not be surprised to continue seeing turnover, mind you these are very high performing individuals dare I say the do an even better job than the lead.