The Best Alternative to a Negotiated Agreement in the context of severance negotiations has a speculative value for both parties, and this fact presents an opportunity for leveraging and interest positioning.
For the employer, a failed severance negotiation means not paying money & other benefits; but it also means not acquiring the (former) employees promises and releases, or perhaps not acquiring a resignation decision by a problem employee. But, employers value predictability and control, and a walk-away BATNA provides neither. They’ll have to wait for the next shoe to drop and suffer some disorganization from an abrupt departure.
The walk-away BATNA for the employee is freedom to file a lawsuit, make an agency complaint, engage in lawful disparagement in social media, perhaps influence future business that might come to that employer. If still employed, it may be an opportunity to ride it out as a disaffected employee while searching for other opportunities. But, employees value new economic rewards, and a walk-away BATNA promises no immediate pay-off. Revenge does not buy groceries, and it has a habit of blowing back.
If the employer is informed of what their walk-away BATNA looks like in comparison to the upside of a mutual agreement delivering predictability, control and continuity at time of separation, the employee gains valuable leverage. If the proposal addresses these underlying concerns and interests of the employer, then this leverage pays off.