why are 120v coming out of my light switch plate screws?
My guess, since everything works fine still, is that the ground to this circuit is disconnected somewhere and the hot or possibly a loose "disconnected" hot from another circuit is touching the switch yoke or box.

If your ground were in working order and connected properly to the box or device, the breaker would have long since tripped. I don't know what mess is going on with the hot or some energized wire, but this will be a problem within it.