The order has to be one word: 'standdown' but testing this on my own I can't get them to move either.
I check for idle units in my map in a different way though, not sure if this will help in your case. If you have a trigger that specifically tells the unit to cast an ability it will become idle afterwards, so what you can do is detect the unit casting the ability and add it to a unit group (it will probably work too if you have a 'unit is attacked' event). Then you have a periodic trigger that pick the units in that unit group, orders them to move and then clear all units from that unit group.