Resigned in this context generally means, person was given an ultimatum, resign or be fired. If fired, that is something much harder to explain away in a job interview for a future position vs a resignation. In both cases, the person is out of work.
I had that ultimatum once, about 26 years ago. It was good that my boss was a kindly man, so he allowed me to do the whole resignation letter thing, making it easier for me to find a successive job.