There are a lot of ways that a boss can alienate their employees to the point that they'd rather face the job hunt again, and some of the worst ones will have workers leave in a mass exodus.
But one of the fastest ways to ensure a worker parts with you on bad terms is to threaten to fire them at every available opportunity. Because while some managers might think this makes people scared to defy them, it actually tells them they can't count on this job and should start looking for another one as soon as possible.
And as one story perfectly illustrates, the employee who has their job threatened every week isn't exactly going to worry about making their departure convenient for the person who did that to them.
