Definitely voltage spikes. It's exactly how a car coil makes thousands of volts, from 12V. Give it 12V, and you get a strong magnetic field, when you disconnect, the collapsing field multiplies the voltage. The solenoid isn't designed to do that, instead using that magnetic field to convert electricity into mechanical force, but I'd still reckon you are hitting the minipac with hundreds of volts! What I don't understand is that pinball machines from what? the late seventies onwards have plenny electronics in them. How were THEY preventing the same spikes? I would have assumed there would be diodes built in to the solenoids. I guess they appear elsewhere in the circuit instead...