With Jack Brennan stepping down as Vanguard's CEO, rumors swirl about what actually drove him to hang up the towel.

But here's a news flash. What if he just wants to go out on top? Is it a crime to retire early when you're in good health and relatively young?

Of course not. At the same time, someone as driven and energetic as Brennan doesn't seem like retirement material, does he?

It does smack of perfect symmetry, however. Brennan became CEO only after John Bogle was forced out. Give him the benefit of any doubt. Let's assume that move was a joint consensus of Vanguard's board and not a move orchestrated covertly by Brennan.

Even so, the company's first major succession was poorly handled. It was a public relations nightmare. As a willing or unwilling participant, Brennan's legacy will always be tied to those series of events.

And what a legacy it is. Bogle brought Vanguard into existence and made it a force. Brennan put it over the top by carrying forward Bogle's shareholders-first philosophy to a wide range of products. It's no small feat that Vanguard is the industry leader in terms of low-cost exchange-traded funds. But it's perhaps more impressive what he's done with actively managed funds, pushing prices to levels that should embarrass the competition.

Some Vanguard shareholders grouse that Brennan brought an MBA attitude to a down-home, earthy firm. But times change and a bit of business polish was no doubt needed in what can be a very cutthroat money management world. Vanguard grew up under Brennan and it'll be interesting to see what direction it goes in the future.

Will McNabb fill out Vanguard's lineup with micro-cap and small-cap international index funds? How 'bout some currency and global ETFs? And will it be able to capitalize on an enviable position of holding a patent over the practice of creating ETFs as second share classes of mutual funds? That would seem to play right into the market's thirst for real actively managed ETF portfolios.

Why would Brennan leave at such an exciting time?

Perhaps the only thing left is to rid himself of the stain from how he got to what certainly must be viewed as a career pinnacle. By leaving now while he's young and at the top, Brennan is making the ultimate in anti-ego statements. Nobody can accuse him of hanging on. More to the point, nobody can say he didn't practice what he preached in the boardroom 12 years ago.

Like him or not, Brennan is certainly leaving on his own terms. Sometimes, that can take more courage than walking out the door after someone's given you the boot.