Unfortunately, I have seen a number of teachers come and go throughout my 20 years in education. And for every teacher who leaves my school, there's a different reason for why they left. However, I would say that I've seen some patterns in the departures of teachers in terms of both why they left and in the manner in which they departed the school.
And reflecting on this—all of which is based on my observation, not on any research—I believe it might be helpful for school leaders and for the teachers who remain to process the departure of their colleagues. Maybe some of these ideas can help the “professional breakup” to go more smoothly than it sometimes does.