I'm thinking the nice salt air in a shipping container that has no vents. Sun hit's it and moisture can't get out so it becomes humid like a low tide swamp, then it gets dark & cools. Repeat this many times on the dock on the ship and I can see how the copper gets attacked. The sad thing is $1.00 worth of Di-electric grease in those fittings would have prevented all failures to date.
The fix is simple although time consuming. Remove switch, disassemble, soak in White Vinegar for 30min or so. Rinse with water, apply di-electric grease and re-assemble. White vinegar will not harm plastic, so you can soak all electrical connectors without pulling the wires out. Rinse/grease/re-connect. Any time I have a connector apart on any vehicle I own it gets vinegar if needed, then grease.
Believe it or not when a Scuba Regulator is rebuilt this is the exact procedure except only silicone grease is used on the o-rings.