Earlier quoted context omitted.
Because they were threatened with their career potentially being ruined with a bad reference?
I never understood this. There's nothing preventing a previous employer from giving someone a bad reference if they were to voluntarily leave either.
Sure, but if a company fires people for cause, its future references might well state that (even more so if its just terminated several people 'for cause' to save severance money and is having to create the appearance of actual performance reviews to try to make its breach of employment law less obvious). If staff 'voluntarily' leave and save the company severance money, there's no incentive for them to make any claims about termination due to underperformance or say anything else unpleasant. In theory, they might even deliver on promises of 'really nice letters of recommendation' to avoid disgruntled ex-employees talking to employment lawyers.
On the other hand, many jurisdictions have laws which make it pretty difficult to fire people for cause without clear evidence of misconduct or performance improvement plans and probationary periods for underperformance, so there's a good chance the termination threat is just a bluff to see who will resign...