A lot of the time, people resign because they aren't happy with their job, and believe there's not much that they can do about it. Maybe they don't like their

A lot of the time, people resign because they aren't happy with their job, and believe there's not much that they can do about it. Maybe they don't like their manager, or some of their colleagues, or think that the work is boring, or the job is not conductive to their future career success, or that there is something systematically unfair going on in term of work load or salary, or so on. People on a team understand they cannot fire team members, even if they are dickheads. They cannot change what they work on, or double their salary. That means there will be radio silence for the last few weeks or months when they are looking for a new job. Employees also tend to think the employer already knows what the problems are. If the problems are not being addressed, they will think the employer does not think they can or should be fixed. Why bring up something the employer obviously must be aware of, but has done nothing about? In my experience, the resignation announcement will, indeed, usually come out of the blue. For an observant manager, it may not become as a total surprise; there are some signs. But almost never will employees discuss with you about these things in earnest. There will be signs, but not forthright discussion.