It's usually something that happens when the plastic (or metal) arm of the flipper button opto gets slightly bent out of whack. The interrupters aren't clearing the one opto. Try adjusting the arm.
The F8 switch is activated when the opto on the little board stops being interrupted. That is to say, the standard state of the opto is the beam from one side not making it to the other. This is so if the opto fails, it doesn't make the game think the flipper button is stuck pressed. When you push the button, the little plastic bit gets moved up, and out of the way of the opto beam. Now the beam can make it from one side to the other, and it registers a button press.