1. Not happy with career stuff at that point anyways, so why keep doing it? My dad liked his job but didn’t like where his job was (nuclear power is bad like that), so he retired a bit early.
2. Your niche ages out (eg a mainframe programmer before they were back in demand again).
3. You simply want to do something else.
4. You are in an industry that pushes out older workers regardless of their desire to keep working.
The majority of people retire when they are forced to, not when they plan to. Everyone has a plan until life punches them in the mouth. The question is what does your life look like after that?
My definition of retirement does not mean stop working. In fact working is encouraged as long as you enjoy it.
I have actually addressed the issue of work on the website, in "What to do after retirement?" section.