There is a small capacitor on the motherboard that holds a charge to keep the date and time correct. It can go bad or simply discharge if the main battery goes dead.
Starting in the mid-1990s, the engineer who designed the controller changed it to reset to his birthday, August 27, 1956. I suspect the chip's firmware has to stuff some number in the register when it reinitializes instead of leaving it as garbage, and it just stuffs in Ray Montagne's birthday instead of zero.
(Ray Montagne also made Control-Command-Power give hard restarts on Macintosh systems - having done great work on the Apple II systems, he brought the equivalent of "Control-Open Apple-Reset" to his ADB keymicro controller for the Apple IIgs, and later Macintosh work, too. Ray has forgotten more about desktop microcontrollers than everyone reading this ever knew, combined.)
In Mac OS X, or perhaps in recent versions of firmware, the reinitialization value is midnight, December 31, 1969, GMT, with a default time zone value of "Cupertino, CA," where Apple is located. I'm guessing again, but I'd bet a nickel that some Unix software recognizes a 1969.12.31 or 1970.01.01 date as "uninitialized" and takes appropriate action, which makes it a more useful default choice than Ray's birthday or 1904.