> What rational HR department would want to keep a manager who makes their subordinates feel unsafe, work ineffectively, etc.?
Any of them, because:
(1) The manager is more likely to know their legal rights,
(2) The manager is more likely to be able to afford the cost of litigation in a dispute,
(3) The manager is more likely to have a contract beyond at-will employment,
(4) Determining whether the manager is a problem or the employee is malingering has a cost, plus risks creating evidence of corporate liability
Etc.
Except in the most clear and egregious cases, HR is naturally aligned with management against the human resources that management manages; that's kind of the point of HR, whose function is to assist management in efficiently (i.e., at minimum cost per unit value produced) managing those the human resources.
HR is not a neutral referee; the closest there is to that is government labor relations authorities.