“What was your reason for leaving that position?”
You're interviewing someone for a job in your company. You're asking him about his previous jobs. He just told you about his last job. You want to know why he stopped working there.
What was your reason for leaving that position?
In business, a person's "position" is his or her job or job title:
My current position is Assistant Director of Sales.
That position has been open for a few months now. I wonder when they're going to hire someone.
The word "position" sounds classier and a little more formal than "job" or "job title".
"What is ___'s reason?" is a way of asking "why". Use this to ask why someone made a decision. For example:
What is their reason for wanting to sell it?
"What is ___'s reason?" sounds more formal than "Why". When you ask someone "Why?" instead in a formal situation, it might seem like you're criticizing their decision:
A: Why are you quitting?
B: (surprised) Well, I... I've simply decided that it's the right time.
"What's your reason for ___ing?" doesn't have the same emotional impact. This is a polite way to talk to a coworker who's leaving your company:
A: May I ask what your reason for quitting was?
B: Well, I guess I'm just burnt out and I feel like I need a change.