too much moisture!!

About a month ago I pulled out our suitcases to pack for a camping trip, only to be greeted by the smell of mold. After inspecting them I found both our suitcases, and about half of our bags had mold on them. I was confused and annoyed, we keep them in our closet, how did they get wet? The only thing I could come up with was the moisture in the air had got trapped in the closet. So I broke down and bought new suitcases (we buy cheap ones from walmart, the ones I threw out we bought 7 years ago for about $30).

I keep our new ones in the closet still, but we make sure to leave the door open, I'm hoping that solves the issue, if not the next step is to buy a dehumidifier, something I don't really feel like purchasing, but will if it's necessary. I've inspected just about everything else in the closet and haven't found any other mildew or mold, so hopefully the bags are the only loss.