Earlier quoted context omitted.
I'm always surprised people can never seem to see these things coming. If I was working there, I would've started looking two or three months ago. Maybe they had some stock options or something they were holding onto??
Maybe they have families and don't want to upset the status quo until it's absolutely necessary.
As a logical exercise, if we assign some risk to your employer terminating you (I imagine especially for publicly traded companies one could come up with some more-accurate-than-guess-formula), then there would be a point at which the risk of your current employer would outweigh the risk of leaving to find another job. Or, another way, the risk-reward between two different jobs could be measured (e.g. working at risky Startup X vs stable Retail Bank Y).
Links welcome!