Donald Trump made his name as a Reality TV star with the trademark line, "You're fired!" As President in the real world, instead of the very unreal Reality TV world, he may have used it one too many times.
There is a lesson here for entrepreneurs. Firing an employee needs to be done at least as well as the initial hiring, maybe better. A firing has greater impact on the rest of the organisation.
Although your conversations one-on-one may be very private, a firing tends to get more public attention and generate stronger reaction. Maybe it's the "fear of firing." The message received by employees staying on the job will be more important than the message delivered to the employee who was fired. Their perceptions and interpretations of what "really" happened will affect their own behaviour.
They have seen how someone got fired, now they need to know why.
Could it just be something the boss didn't like? Something that was said or done that had nothing to do with the job?
Be sure they get the right message, and quickly.