There's three things that cause turnover:
1. Player boredom with their character(s), usually but not always leading to one of the next two options.
2. In-game events, i.e. simply through doing what it does and following its own personality a character in effect role-plays its way out of the party. (I've done this with characters I wanted to keep playing, just because it made sense from the character's perspective)
3. Character death, without revival. (whether the player is bored with the character or not)
As a player, I've done all three, many many times each.

As a DM, I've also seen each happen a great many times. I'm cool with it, and don't see it as a problem; as long as there's at least some sort of identifyable party left when they're done. And sometimes, retired characters return later and become significant.
What I *have* learned is to not build a story or plot around any particular character, as for sure that'll be the one that retires or dies next.
The general rule-of-thumb I use is that new characters come in a level below the party average, ignoring NPCs, henches, etc.
Lanefan