Hi, I've only ever had this happen after the cookies had sat for weeks or months under very dry winter conditions. It sounds like you're overbaking or over-drying the icing, and/or storing them under extremely dry conditions. Are you dehydrating them to set the icing? If so, maybe don't do this and/or bake your cookies for a shorter time, and/or introduce some moisture into your ambient environment (run a humidifier?).