That's the wierd thing about BIOS settings, they stay static for years and then one day we find that somethings changed but we NEVER did it!!
it's all to do with the way that power is supplied to the board. If the supplies values change drastically for some unknown reason it can cause the saved "charge", inside the memory that has stored it, to look different.
The memory stores a Voltage inside it to indicate a 0 or 1.
If suddenly the supplied voltage to the chip increases then the stored charge looks less, so logically it looks OFF or 0.
If the supplied voltage to the chip decreases then it looks more so it appears logically as ON or 1
look at this VERY SIMPLE (maybe oversimplified) diagram
the vertical direction is the Voltage from the supply
the red line is the charge inside the IC
the grey lines are the upper and lower boundaries of tolerance for determing whether a charge is false true or in no mans land!
The blue diagonal is what the Voltage from the supply COULD be .
when the blue line is low, the red line is considered to be logical 1
when the blue line is high the red line is considered to be logical 0
Since the grey lines will also shift proportinally with the blue line there will be a time when the logic level could be either a 0 or a 1
the red line is static because it is like a small battery inside the chip that should never lose its value unless someone specifically defines that the value should change.
like I said it's greatly oversimplified .. but you'll get the idea. That;s why it's so critical to have stable, noise free (electrically) power supply rails.