This is the best
explaination I've heard:
If you
stick your head into the
tub of a
top-loading washing machine and look up, you will notice that there is a
gap between the tub and the
housing of the machine. The gap is there so that the tub can move independently of the housing, during the
spin cycle, etc. If your machine fills with too much water, or if you put too many clothes in the machine, the
water line can come
dangerously close to this gap. If one of your socks is near the top of the load, and the other is towards the bottom, the top one might get tossed over during one of the
cycles.
Of course this probably doesn't happen only for socks, you just don't buy your
underwear in
pairs.