i think it could be the oxidizing reactions that causes the cards to turn yellow but i could be wrong.
No, you've got it right about the oxidation. The chemical compound in paper that oxidizes to give the yellowish tint is called lignin. The more lignin your paper has in it, the more rapidly your paper will yellow. Sunlight and air in general can speed up this process. It will take a while for playing cards to experience this effect due to the chemical composition of the paper used; however it will likely happen
eventually whether you like it or not.
So to answer your original question Frost, no, humidity does not turn your playing cards yellow. Yes they can still turn yellow while sealed in plastic because a deck seal is not an airtight seal; however, the cellophane will extend the period of time before this occurs significantly, as will keeping your cards out of the sun.
I hope this answers pretty much everything you ever wanted to know about your cards changing colors.