Most fuel caps are set up to hold air pressure in the system, and allow air in when there is reduced pressure in the tank. This is to prevent fuel vapors from escaping the fuel system and getting into the atmosphere. Generally there will be a method for getting the fuel vapors to the engine. In gasoline engined cars, this is a charcoal canister which has a vacuum hose going to the intake manifold. There is another line going to the tank with a check valve, so that vapors stay in the tank unless the engine is running to draw the fumes into the engine and burn them.
This time of year, it's not unusual to see air pressure build up when you have a cold overnight, making the fuel system 'breathe in' through the cap, but can't 'breathe out' the next (warmer) day due to its design. When you remove the cap it lets out that warmed-up air that's built up some pressure.
The problem generally will be worse when the tank is near empty, since a cold tank overnight will take in more air which will expand over the day. A tank that's mostly full won't have room for air, and the fuel won't expand or evaporate much to cause the pressure to build up.
With the CUCV's being mostly 'off the shelf' GM trucks, I'd expect that their fuel caps are the same as the diesel caps for civi vehicles, so they'd be designed to meet the Clean Air Act specification. I'm not sure about the rest of the fuel system, since I don't have a CUCV myself to compare with.
