I'd mount the switch on the playfield again and look closely to make sure the switches aren't getting mashed together as you install it. Could be a part of the pop bumper mech catching the end of a switch leaf (and forcing it to close the switch). If it was working ok before and all you did is replace the switch (and you're certain the wiring is correct), then I'd have to assume it's the way the switch was mounted. The new switch leaves may be a bit longer, or perhaps a leaf got caught by a nearby part while installing it. Hopefully it's an easy fix.

The transistor is on the pop bumper driver board.