Why do people develop loyalty toward the groups they join? Evolution? Reciprocity? Indoctrination?

Either way, not unusual. Happens all the time in the military ("no such thing as an ex-marine"), around football clubs ("once a red, always a red")... pretty much everywhere there are humans, especially men in high-pressure environments.

Here is one example: WSJ: http://www.wsj.com/articles/SB124139546243981801

"During that time, the New York Fed's chairman, Stephen Friedman, sat on Goldman's board and had a large holding in Goldman stock, which because of Goldman's new status as a bank holding company was a violation of Federal Reserve policy."

It's an old boys club plain and simple. You don't get to high places without friends and in turn you scratch their backs.

In the world of regular employment, usually they don't. But in the tight-knit circle of CXO-level personnel of major financial institutions that may not be the case. It strikes me as a club, and to stay in the club one must help other members of the club and practice a bit of "omertà."

They're helping their friends at the time, and helping themselves when they decide to go back to work at those firms.