Any intense heat in that cabinet? Maybe the glossy paper used on wii games has melted on the surface and caused color to shift. Thats my only guess. But alas im not sure I've seen yellowing of all manner. And fading of print ink. Early on collecting I placed giant wool blankets over my windows. With the lights off they get minimal light at all. And they seem ok.
Idk how on earth that could happen in a dark closet. I'm also fairly sure moisture wouldn't cause it. no warping. No splits or creasing. If the moisture was exteme enough to soak the whole paper. Itd have to have done all of them as well as manuals. Warping. Ive seen white inserts as old as 10 to 30 yrs old with warping and little cracks from moisture.
Is it possible the games had that before you put them in there and you just didn't recall back then?
Also one thing I found out that mind blew me. Is that LED bulbs can cause fading. I always thought it was only sun. If you have an LED bulb close enough to games or cardboard. On constantly. It will fade the area somehow. Happened to some of my funko pops. Had them under a string rope light around my ceiling so they would glow from under. Pretty crazy. Happy I learned fast.
I reccomend moisture bags or a dehumidifier all together. Put some baking soda in the closet. Some silica packets in the cabinet and replace yearly. About 10 bucks for some on amazon.
Best of luck. Hopefully someone helps you find the culprit