That's been my experience in academia as well: any person leaving can be rationalized somehow, and only individual staff personally invested in that person tend to do any introspection about what could've been done differently. Multiple departures can cause a shake-up, though, mostly because the higher-up administration that usually ignores such matters may feel the need to investigate if a trend seems to be developing. Of course, if the problem is the higher-up administration's own policies, that doesn't help, but it can help if the problem was unnoticed malaise in a department.