Have you ever noticed that when you turn the switch toggle on or off that it kind of "snaps" into the desired position in the last few 10ths of an inch of movement? Well, this is designed into the switch. When you move the toggle it moves a spring loaded switching mechanism that snaps to the other position. The purpose of the spring load is to keep you from s l o w l y moving the switch from one position to the other which is when arcing can occur. My guess is that you probably moved the switch to the off position but not quite far enough to take it out of the spring loaded position. After a short while it simply moved itself into the on position to satisfy the spring tension you were unaware that you had engaged but not completed....