All kinds of reasons for it.
1. Coach plays your DD to much. (Overuse concerns)
2. Coach plays your DD to little. (No playing time concerns)
3. Teams goals are not the same as yours. Not playing the level you want. ie..18U vs 18G, or USSA or Triple Crown vs ASA.
4. Coach is a nut job.
5. DD can't learn from the coach. (Coach is truly in over his head).
6. Coach is very emotional. (a screamer or challenges umpires to fight in the parking lot during games).
7. Drive is to far.
8. Your DD would like a shot at a different position and coach refuses to even look at her or even give her a tryout. Then advertises for the spot constantly. (The Rodney Dangerfield Syndrome or "no respect").
9. Daddy ball.
10. You don't like the gang member parents on the team.
11. Dues and fees are to high.
12. Bad team chemistry. (the eye roller syndrome)
13. Another team offered you a full ride. (No dues or fees some even pay travel).
14. Your DD is not as talented as the others on the team and feels she is holding the team back but wants to continue to play. Leaving for a lower level team. (The rec player in travel effect).
15. Your DD is the team stud and playing with the team is keeping her from improving her skills. (The travel player in rec ball effect).
16. Team dissolves and you are forced to find a different team.
17. Coach refuses to improve the team. Keeping the same poor (not improving) players season after season.
18. Coach writes a postcard to your DD calling her a sweetheart! LOL
19. Players are mean to each other.
20. DD is an Honor Student who requires much time to do homework and coach likes to practice every night for 6 hours.
21. Parent thinks DD should be SS and bat clean up when DD is the worst player on the team. (The rose color glasses syndrome).
22. DD is cut from her team.
23. The moms on the other team are better looking.
24. The mom who runs the other teams snack bar makes a mean tri-tip.
25. Coach for the new team owns a pole barn.
26. Coach is misdirecting team funds.
27. Coach bad mouths your DD on a national board.
28. Parent simply believes the grass in greener with the new team.
29. New team asks you to help coach. (ego effect).
30. Coach forces DD to play while injured.
31. Parents are nut jobs.
It really doesn't matter what the reason. For coaches it will always be a sore subject. Especially when it happens to them a lot. Parents will always be the "shot callers" and will do what they think best for their child. Coaches should understand this and try not to get all butt hurt over it when a player leaves for what ever reason. It seems to me that the ones who take it the worst are the ones who have constant problems with girls leaving them. They are the ones who should take a serious look in the mirror.
As far as why it doesn't happen in Little League as much as Fastpitch? Well, just watch the Little League World Series one time. Lets face it, the talent level is not even close to the same. Any Fastpitch rec ball team could beat the winner of the Little League World Series.