We get one every few years. It's usually 1 of 2 things:
1. Troop isn't active enough; scout looking for more outings/activities. No problem, we are active.
2. Some other alleged behavioral issue in the troop(poor communication, lack of opportunity for advancement, scout doesn't get along with others). These issues that they blame on the old troop almost always repeat themselves in our troop, as the problem isn't the troop, it's the scout or parent. It's easier to blame the troop than take personal responsibility.