The criteria comes from the scientific method. Take some data about people that left, people that didn't, build a model. It will be wrong, gather more data, tweak, etc.

Will managers know first? They should, but they might now. People keep lots of stuff from managers that might come out from a model. And some people might not fit the model, but the manager knows they're quitting.

It's just another tool, and I think a valid one. Not necessarily the main one, but one to assist a manager.