The SSA has a death index. http://en.wikipedia.org/wiki/Social_Security_Death_Index Banks probably register your SS number (which they are required to have) with the SSA for "notify if dead" (just a guess). Doubtful they give every financial institution the SS numbers of all deaths on a regular basis. Then again they might have direct access to such information since they can't open up an account for a dead person.
Of course it suck if you end up on it while you are still alive
>She learned that she was dead when she received a letter from her bank. The bank expressed its corporate condolences, and locked her out of her account. The Social Security Administration also stopped her retirement checks, and her health insurance also stopped.