Contacts too close on the pop bumper. Check under playfield and adjust gap. Sounds like vibration is causing contacts to join and randomly activate the pop bumper.
+1
Easy test is to thump the playfield with your fist in the vicinity. Most likely the vibration from an adjacent pop bumper.
could also be the switch matrix capacitor across the switch causing the pop bumper to activate. Easy test is to snip one side of the switch capacitor (mounted on the switch). If the problem goes away - it is a faulty capacitor.
