There are all sorts of reasons why staff leave, but here are some positive ones:
A Stage in Life
Sometimes staff leave because they have to take care of an aging parent, the desire to go back to school, marriage or even retirement.
A Stirring of God
Sometimes staff change ministry venues because God simply calls them to another mission field. 20 percent of Americans move every year. It's no wonder that pastors will often make transitions too.
A Season of Healing
Sometimes staff have to take a break from ministry in order to focus on their family, a marriage or even a wayward child. That's biblical. Ministry can be grueling. It can overtake a persons life and leave them drained. Some times staff need to take a break because of physical or emotional issues.
A Situation of Privacy
On rare occasions, some staff leave ministry due to private matters. In those cases, I've tried to protect their privacy and not publicize why someone is leaving.