Help on confectioners sugar!!!!!!!!!?

I bakes some crinkles recently and this is what I did. I chilled the mixture for about 3 hours then rolled them into then covered them with the sugar. Then I popped them in the oven then let it chill. Later, I placed them into plastic containers.But a day later. I noticed it started to water and moisten.. HOW DID THIS HAPPEN????.... WHAT can I do to prevent this.???