I own several Bally EM games and I have a guess. I bet your playfield was pushing against the left flipper button leaf switches and two blades that enable the lock relay were constantly closed. There are two sets of blades on the left button; one for flippers and one for the lock relay. The playfield can push on the blades, but not enough to engage the flipper switches. However, enough to close the blades for the lock relay. Therefore, when you turned the power switch on, the machine automatically 'locked' on.