You had the case open, were probably manipulating the board and parts somehow; a bit of stress on a weak solder joint or a shifting of some bit of conductive debris so it touched something critical, are two other ways it could have 'picked that moment'.
In the late 1970s, the newfangled MOS was determined to have sensitivity to small charges that could kill it during handling or even storage. In the 80s, lots of redesigns happened, to add protection. In the 90s, unprotected designs became very rare. Fully-assembled circuits (all the potentially sensitive inputs connected to insensitive conductive output circuits) aren't likely to fail due to static, not today.