The iPhone is on top of the world. Why are Apple execs exiting?

Apple CEO Tim Cook should be celebrating, and not just because he turned 65 in November. Instead, he's beset by questions about a retirement he hasn't announced yet — and stories about his executives heading for the exits.

We'll get to the who, but first of all: Why? Why would anyone with executive-level stock options want to leave Apple right now?

Cook's company may not be be the world's most valuable (after a remarkable run), but it has been growing steadily and is worth more than $4 trillion ...