Some of the places will get rid of girls that provide poor quality service, irritate customers or create some sort of controversy. The establishment doesn't want people to 'tarnish' their image. I noticed that Lily's got rid of a couple of girls that had annoyed customers. I am sure that the other places have done the same.
The girls also find that they fit in with the vibe of the place, or the commuting is easy, etc. These are all things which I have heard of. I am sure that it is the tip of the iceberg. Every girl, every place has their own individual reasons.