Julia M. Usher posted:I call this "sheeting" - when the icing slides off the cookie surface in one big sheet. This has happened to me in the past when both the cookie and icing have been extremely dry, usually after having been stored under super cold and dry conditions for a relatively long period of time. If your cookie recipe bakes dry, rather than soft, then this, too, could cause the icing to "sheet" faster than usual. It sounds like your cookies may have been dried out, since you mention having had them stored for "several days".
Agree one hundred percent on Julia's explanation. Same exact reason this happened to me was due to the weather and humidity inside and the cookies/icing was too dry. In very cold weather, like now, I tend to bake a minute or two less than usual and have escaped this problem...thus far...you never know with the heat blasting through the room. Good luck.