I use plastic clamshells and one thing I've noticed is that they can retain heat, and any excess warmth can cause condensation to form inside the plastic container which can moisten and un-crust frosting. This is due to the fact that clamshells are air-tight and can't stay acclimated to the temperature of the room as easily as boxes which have more ventilation. So, if a clamshell full of cupcakes is in a room where a sunbeam from the window can hit it, or where warmth from the oven or stove can seep into it, or even if you don't have your AC on in the car during transport, etc, the change in temp can cause little condensation droplets to form inside (like on the outside of a cup of water or can of pop). These little droplets can moisten and un-crust icing.
I combat this by making sure my cupcake-filled clamshells are kept out of the sun in a shady area or cabinet, and if I have to keep them around for a day or two I keep them in the basement where it's slightly cooler.